Loans and Allowance for Credit Losses - Schedule of Allowance for Credit Losses and Recorded Investment in Financing Receivables (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Allowance for Loan Losses [Roll Forward]
Allowance for Credit Losses, Beginning balance	$ 24,353	$ 17,162	$ 17,280	$ 19,969	$ 19,969
Allowance established for acquired PCD loans			1,464
Charge-offs	(133)	(51)	(304)	(163)	(356)
Recoveries	24	22	64	94
(Reversal of) provisions	(584)	81	5,156	(2,686)
Allowance for Credit Losses, Ending balance	23,660	17,214	23,660	17,214	17,280
Financing Receivable Excluding Unfunded Commitments
Allowance for Loan Losses [Roll Forward]
(Reversal of) provisions	(584)	81	5,156	(2,686)
Commercial Real Estate
Allowance for Loan Losses [Roll Forward]
Allowance for Credit Losses, Beginning balance	13,037	10,196	10,578	12,010	12,010
Allowance established for acquired PCD loans			798
Charge-offs	(32)	0	(32)	0
Recoveries	0	0	0	0
Allowance for Credit Losses, Ending balance	12,980	10,392	12,980	10,392	10,578
Commercial Real Estate \| Financing Receivable Excluding Unfunded Commitments
Allowance for Loan Losses [Roll Forward]
(Reversal of) provisions	(25)	196	1,636	(1,618)
Residential Mortgage
Allowance for Loan Losses [Roll Forward]
Allowance for Credit Losses, Beginning balance	5,204	2,839	2,976	3,303	3,303
Allowance established for acquired PCD loans			140
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Allowance for Credit Losses, Ending balance	5,224	2,870	5,224	2,870	2,976
Residential Mortgage \| Financing Receivable Excluding Unfunded Commitments
Allowance for Loan Losses [Roll Forward]
(Reversal of) provisions	20	31	2,108	(433)
Commercial and Industrial
Allowance for Loan Losses [Roll Forward]
Allowance for Credit Losses, Beginning balance	2,237	1,465	1,416	2,048	2,048
Allowance established for acquired PCD loans			194
Charge-offs	0	0	(14)	0
Recoveries	1	6	11	24
Allowance for Credit Losses, Ending balance	2,035	1,772	2,035	1,772	1,416
Commercial and Industrial \| Financing Receivable Excluding Unfunded Commitments
Allowance for Loan Losses [Roll Forward]
(Reversal of) provisions	(203)	301	428	(300)
Home Equity Lines of Credit
Allowance for Loan Losses [Roll Forward]
Allowance for Credit Losses, Beginning balance	450	312	294	397	397
Allowance established for acquired PCD loans			13
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Allowance for Credit Losses, Ending balance	410	296	410	296	294
Home Equity Lines of Credit \| Financing Receivable Excluding Unfunded Commitments
Allowance for Loan Losses [Roll Forward]
(Reversal of) provisions	(40)	(16)	103	(101)
Real Estate Construction
Allowance for Loan Losses [Roll Forward]
Allowance for Credit Losses, Beginning balance	3,256	2,247	1,918	2,070	2,070
Allowance established for acquired PCD loans			169
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Allowance for Credit Losses, Ending balance	2,855	1,780	2,855	1,780	1,918
Real Estate Construction \| Financing Receivable Excluding Unfunded Commitments
Allowance for Loan Losses [Roll Forward]
(Reversal of) provisions	(401)	(467)	768	(290)
Consumer
Allowance for Loan Losses [Roll Forward]
Allowance for Credit Losses, Beginning balance	169	103	98	141	141
Allowance established for acquired PCD loans			150
Charge-offs	(101)	(51)	(258)	(163)
Recoveries	23	16	53	70
Allowance for Credit Losses, Ending balance	156	104	156	104	$ 98
Consumer \| Financing Receivable Excluding Unfunded Commitments
Allowance for Loan Losses [Roll Forward]
(Reversal of) provisions	$ 65	$ 36	$ 113	$ 56

ACNB CORP 10-Q