Allowance for Credit Losses - Aging of the Recorded Investment in Loans by Past Due Category and Class of Loan (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	$ 2,610,814	$ 2,514,917
Financing Receivable Recorded Investment	3,982,799	3,852,432
Commercial & Industrial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	589,899	570,638
Financing Receivable Recorded Investment	592,061	572,966
Farmland \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	125,082	129,934
Financing Receivable Recorded Investment	128,166	132,986
Non Farm, Non Residential \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	882,215	810,392
Financing Receivable Recorded Investment	884,711	812,871
Agriculture \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	129,248	146,054
Financing Receivable Recorded Investment	132,569	149,289
All Other Commercial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	555,618	539,333
Financing Receivable Recorded Investment	558,214	542,397
First Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	459,375	450,232
Financing Receivable Recorded Investment	460,966	451,681
Home Equity \| Residential
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	100,393	89,498
Financing Receivable Recorded Investment	100,718	89,839
Junior Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	70,465	64,712
Financing Receivable Recorded Investment	70,675	64,901
Multifamily \| Residential
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	328,752	318,566
Financing Receivable Recorded Investment	330,183	319,711
All Other Residential \| Residential
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	38,930	44,378
Financing Receivable Recorded Investment	39,081	44,610
Motor Vehicle \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	653,308	636,572
Financing Receivable Recorded Investment	656,174	639,538
All Other Consumer \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Loans receivable	29,130	31,486
Financing Receivable Recorded Investment	29,281	31,643
30-59 Days Past Due
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	12,771	20,844
30-59 Days Past Due \| Commercial & Industrial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	3,378	746
30-59 Days Past Due \| Farmland \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment		598
30-59 Days Past Due \| Non Farm, Non Residential \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	220	1,619
30-59 Days Past Due \| Agriculture \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	75
30-59 Days Past Due \| All Other Commercial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment		1,297
30-59 Days Past Due \| First Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	818	4,304
30-59 Days Past Due \| Home Equity \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	599	639
30-59 Days Past Due \| Junior Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	443	356
30-59 Days Past Due \| Multifamily \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	706	529
30-59 Days Past Due \| All Other Residential \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment		25
30-59 Days Past Due \| Motor Vehicle \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	6,132	10,176
30-59 Days Past Due \| All Other Consumer \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	400	555
60-89 Days Past Due
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	2,910	4,170
60-89 Days Past Due \| Commercial & Industrial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	58	768
60-89 Days Past Due \| Agriculture \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	328
60-89 Days Past Due \| All Other Commercial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	64	152
60-89 Days Past Due \| First Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	437	1,361
60-89 Days Past Due \| Home Equity \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	99	157
60-89 Days Past Due \| Junior Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	231	101
60-89 Days Past Due \| Multifamily \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment		74
60-89 Days Past Due \| Motor Vehicle \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	1,575	1,435
60-89 Days Past Due \| All Other Consumer \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	118	122
90 Days and Greater Past Due
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	8,847	5,460
90 Days and Greater Past Due \| Commercial & Industrial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	974	208
90 Days and Greater Past Due \| Farmland \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	177	806
90 Days and Greater Past Due \| Agriculture \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	231	642
90 Days and Greater Past Due \| All Other Commercial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	1,567
90 Days and Greater Past Due \| First Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	1,067	1,224
90 Days and Greater Past Due \| Home Equity \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	633	906
90 Days and Greater Past Due \| Junior Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	200	290
90 Days and Greater Past Due \| Multifamily \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	253	345
90 Days and Greater Past Due \| All Other Residential \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	2,952	108
90 Days and Greater Past Due \| Motor Vehicle \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	761	808
90 Days and Greater Past Due \| All Other Consumer \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	32	123
Total Past Due
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	24,528	30,474
Total Past Due \| Commercial & Industrial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	4,410	1,722
Total Past Due \| Farmland \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	177	1,404
Total Past Due \| Non Farm, Non Residential \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	220	1,619
Total Past Due \| Agriculture \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	634	642
Total Past Due \| All Other Commercial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	1,631	1,449
Total Past Due \| First Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	2,322	6,889
Total Past Due \| Home Equity \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	1,331	1,702
Total Past Due \| Junior Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	874	747
Total Past Due \| Multifamily \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	959	948
Total Past Due \| All Other Residential \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	2,952	133
Total Past Due \| Motor Vehicle \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	8,468	12,419
Total Past Due \| All Other Consumer \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	550	800
Current
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	3,958,271	3,821,958
Current \| Commercial & Industrial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	587,651	571,244
Current \| Farmland \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	127,989	131,582
Current \| Non Farm, Non Residential \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	884,491	811,252
Current \| Agriculture \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	131,935	148,647
Current \| All Other Commercial \| Commercial
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	556,583	540,948
Current \| First Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	458,644	444,792
Current \| Home Equity \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	99,387	88,137
Current \| Junior Liens \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	69,801	64,154
Current \| Multifamily \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	329,224	318,763
Current \| All Other Residential \| Residential
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	36,129	44,477
Current \| Motor Vehicle \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	647,706	627,119
Current \| All Other Consumer \| Consumer
Aging of recorded investment in loans by past due category and class of loans
Financing Receivable Recorded Investment	$ 28,731	$ 30,843

FIRST FINANCIAL CORP /IN/ 10-Q