Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	$ 91,164		$ 91,756
Financing Receivable, Nonaccrual Unpaid Principal Balance	118,398		118,901
Financing Receivable, Nonaccrual Related Allowance	13,984		9,836
Nonperforming Financing Receivable, Average Recorded Investment	90,546	$ 57,849
Nonperforming Financing Receivable, Interest Income, Cash Basis Method	47	387
With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	51,308		53,656
Financing Receivable, Nonaccrual Unpaid Principal Balance	76,419		70,621
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	64,248	37,775
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	47	387
With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	39,856		38,100
Financing Receivable, Nonaccrual Unpaid Principal Balance	41,979		48,280
Financing Receivable, Nonaccrual Related Allowance	13,984		9,836
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	26,298	20,074
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Commercial, financial, agricultural and other [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	23,870		22,422
Financing Receivable, Nonaccrual Unpaid Principal Balance	41,559		31,583
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	29,643	8,684
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	12	78
Commercial, financial, agricultural and other [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	21,290		24,196
Financing Receivable, Nonaccrual Unpaid Principal Balance	23,057		34,249
Financing Receivable, Nonaccrual Related Allowance	11,229		6,959
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	15,685	7,190
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Construction \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		1,475
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		1,475
Construction \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		0
Financing Receivable, Nonaccrual Related Allowance	0		0
Residential real estate [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	10,745		11,874
Financing Receivable, Nonaccrual Unpaid Principal Balance	12,219		13,678
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	11,025	10,122
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	27	19
Residential real estate [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	2,486		1,145
Financing Receivable, Nonaccrual Unpaid Principal Balance	2,782		1,160
Financing Receivable, Nonaccrual Related Allowance	209		162
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	2,332	2,354
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Commercial Real Estate [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	16,684		17,853
Financing Receivable, Nonaccrual Unpaid Principal Balance	22,609		23,807
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	22,570	16,849
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	8	184
Commercial Real Estate [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	16,080		12,759
Financing Receivable, Nonaccrual Unpaid Principal Balance	16,140		12,871
Financing Receivable, Nonaccrual Related Allowance	2,546		2,715
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	8,281	10,530
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Loans to individuals [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	9		32
Financing Receivable, Nonaccrual Unpaid Principal Balance	32		78
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	27	269
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	0
Loans to individuals [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		0
Financing Receivable, Nonaccrual Related Allowance	0		0
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	0	0
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Real estate construction [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	983	1,851
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	106
Real estate construction [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	0	0
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Time and Demand Loans \| Commercial, financial, agricultural and other [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	21,900		21,092
Financing Receivable, Nonaccrual Unpaid Principal Balance	39,589		30,253
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	27,755	7,932
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	12	78
Time and Demand Loans \| Commercial, financial, agricultural and other [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	21,290		24,196
Financing Receivable, Nonaccrual Unpaid Principal Balance	23,057		34,249
Financing Receivable, Nonaccrual Related Allowance	11,229		6,959
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	15,685	7,103
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Senior Lien \| Residential real estate [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	7,190		7,219
Financing Receivable, Nonaccrual Unpaid Principal Balance	8,055		8,148
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	7,504	7,245
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	27	19
Senior Lien \| Residential real estate [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,340		1,145
Financing Receivable, Nonaccrual Unpaid Principal Balance	1,358		1,160
Financing Receivable, Nonaccrual Related Allowance	203		162
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	1,186	756
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Junior Lien \| Residential real estate [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3,555		4,655
Financing Receivable, Nonaccrual Unpaid Principal Balance	4,164		5,530
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	3,521	2,877
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	0
Junior Lien \| Residential real estate [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,146		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	1,424		0
Financing Receivable, Nonaccrual Related Allowance	6		0
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	1,146	1,598
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Multifamily \| Commercial Real Estate [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	10		10
Financing Receivable, Nonaccrual Unpaid Principal Balance	12		12
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	10	20
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	0
Multifamily \| Commercial Real Estate [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	11,699		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	11,700		0
Financing Receivable, Nonaccrual Related Allowance	2,160		0
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	3,900	0
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Nonowner Occupied Commercial Real Estate \| Commercial Real Estate [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	9,288		8,799
Financing Receivable, Nonaccrual Unpaid Principal Balance	13,401		12,879
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	12,266	13,887
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	184
Nonowner Occupied Commercial Real Estate \| Commercial Real Estate [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		5,357
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		5,357
Financing Receivable, Nonaccrual Related Allowance	0		2,280
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	0	4,274
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Owner Occupied Commercial Real Estate \| Commercial Real Estate [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	7,386		9,044
Financing Receivable, Nonaccrual Unpaid Principal Balance	9,196		10,916
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	10,294	2,942
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	8	0
Owner Occupied Commercial Real Estate \| Commercial Real Estate [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,381		7,402
Financing Receivable, Nonaccrual Unpaid Principal Balance	4,440		7,514
Financing Receivable, Nonaccrual Related Allowance	386		435
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	4,381	6,256
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Automobiles \| Loans to individuals [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	7		30
Financing Receivable, Nonaccrual Unpaid Principal Balance	16		62
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	16	198
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	0
Automobiles \| Loans to individuals [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		0
Financing Receivable, Nonaccrual Related Allowance	0		0
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	0	0
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Other Consumer Loans \| Loans to individuals [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	2		2
Financing Receivable, Nonaccrual Unpaid Principal Balance	16		16
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	11	71
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	0
Other Consumer Loans \| Loans to individuals [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		0
Financing Receivable, Nonaccrual Related Allowance	0		0
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	0	0
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Construction Residential \| Construction \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		0
Construction Residential \| Construction \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		0
Financing Receivable, Nonaccrual Related Allowance	0		0
Construction Residential \| Real estate construction [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	0	0
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	0
Construction Residential \| Real estate construction [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	0	0
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Construction Other \| Construction \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		1,475
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		1,475
Construction Other \| Construction \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		0
Financing Receivable, Nonaccrual Related Allowance	0		0
Construction Other \| Real estate construction [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	983	1,851
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	106
Construction Other \| Real estate construction [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	0	0
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Time and Demand Loans Other \| Commercial, financial, agricultural and other [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		0
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	0	0
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	0
Time and Demand Loans Other \| Commercial, financial, agricultural and other [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		0
Financing Receivable, Nonaccrual Related Allowance	0		0
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	0	0
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	0	0
Equipment Finance \| Commercial, financial, agricultural and other [Member] \| With No Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,970		1,330
Financing Receivable, Nonaccrual Unpaid Principal Balance	1,970		1,330
Nonperforming Financing Receivable With No Related Allowance Average Recorded Investment	1,888	752
Nonperforming Financing Receivable, with No Related Allowance, Interest Income, Cash Basis Method	0	0
Equipment Finance \| Commercial, financial, agricultural and other [Member] \| With a Related Allowance
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0		0
Financing Receivable, Nonaccrual Unpaid Principal Balance	0		0
Financing Receivable, Nonaccrual Related Allowance	0		$ 0
Nonperforming Financing Receivable, with A Related Allowance, Average Recorded Investment	0	87
Nonperforming Financing Receivable, with Related Allowance, Interest Income, Cash Basis Method	$ 0	$ 0

FIRST COMMONWEALTH FINANCIAL CORP /PA/ 10-Q