Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	$ 632,688	$ 600,463	$ 529,998
Interest expense	(206,601)	(221,571)	(144,322)
Other income	9,604	9,135	8,087
Applicable income tax (provision) benefits	(39,056)	(35,636)	(40,492)
Net Income	152,302	142,572	157,063
Depreciation and amortization	6,626	5,688	4,104
Other net	10,173	(3,988)	1,665
Cash Provided by (Used in) Operating Activity, Including Discontinued Operation	187,540	129,463	150,759
Cash Provided by (Used in) Investing Activity, Including Discontinued Operation	(388,226)	(121,723)	(574,273)
Payments of Dividends	(55,489)	(52,602)	(50,814)
Proceeds from reissuance of treasury stock	245	204	245
Payments for Repurchase of Common Stock	(35,792)	(12,630)	(14,965)
Cash Provided by (Used in) Financing Activity, Including Discontinued Operation	247,639	(21,324)	416,263
Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation	46,953	(13,584)	(7,251)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	197	242	234
Dividends from subsidiaries	105,273	75,227	107,683
Interest expense	(4,537)	(4,098)	(3,656)
Other income	0	0	0
Operating expense	(5,971)	(5,841)	(6,631)
Income (loss) before taxes and equity in undistributed earnings of subsidiaries	94,962	65,530	97,630
Applicable income tax (provision) benefits	2,088	2,018	2,087
Income (loss) before equity in undistributed earnings of subsidiaries	97,050	67,548	99,717
Equity in undistributed (loss) earnings of subsidiaries	55,252	75,024	57,346
Net Income	152,302	142,572	157,063
Depreciation and amortization	290	300	320
Gain (Loss) on Disposition of Assets for Financial Service Operations	0	0	0
Increase (Decrease) in Prepaid Taxes	(106)	19,809	(20,307)
Retained Earnings Undistributed Earning From Equity Method Investees	(55,252)	(75,024)	(57,346)
Other net	3,054	4,778	7,151
Cash Provided by (Used in) Operating Activity, Including Discontinued Operation	100,353	92,435	86,881
Payments for (Proceeds from) Loans Receivable	3	2	2
Payments to Acquire Property and Equipment	(91)	(609)	(23)
Payments to Acquire Additional Interest in Subsidiaries	38	0	1,120
Payments to Acquire Businesses and Interest in Affiliates	19	14	18
Cash Provided by (Used in) Investing Activity, Including Discontinued Operation	(69)	(621)	1,081
Payments of Dividends	(55,489)	(52,602)	(50,814)
Proceeds from reissuance of treasury stock	245	204	245
Payments for Repurchase of Common Stock	(35,792)	(12,630)	(14,965)
Cash Provided by (Used in) Financing Activity, Including Discontinued Operation	(91,036)	(65,028)	(65,534)
Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation	9,248	26,786	22,428
Cash and Cash Equivalent	$ 96,157	$ 86,909	$ 60,123	$ 37,695