Condensed Financial Information of First Commonwealth Financial Corporation (parent company only) - Statements of Cash Flow (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Operating Activities
Net Income	$ 152,302	$ 142,572	$ 157,063
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	6,626	5,688	4,104
Other net	10,173	(3,988)	1,665
Net cash provided by operating activities	187,540	129,463	150,759
Investing Activities
Cash Provided by (Used in) Investing Activity, Including Discontinued Operation	(388,226)	(121,723)	(574,273)
Financing Activities
Dividends paid	(55,489)	(52,602)	(50,814)
Proceeds from reissuance of treasury stock	245	204	245
Purchase of treasury stock	(35,792)	(12,630)	(14,965)
Net cash provided by (used in) financing activities	247,639	(21,324)	416,263
Net increase (decrease) in cash and cash equivalents	46,953	(13,584)	(7,251)
Parent Company [Member]
Operating Activities
Net Income	152,302	142,572	157,063
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	290	300	320
Net gains (losses) on sales of assets	0	0	0
Decrease (increase) in prepaid income taxes	(106)	19,809	(20,307)
Undistributed equity in subsidiaries	(55,252)	(75,024)	(57,346)
Proceeds from Equity Method Investment, Distribution	65	0	0
Other net	3,054	4,778	7,151
Net cash provided by operating activities	100,353	92,435	86,881
Investing Activities
Net change in loans	3	2	2
Payments to Acquire Property and Equipment	(91)	(609)	(23)
Cash Provided by (Used in) Investing Activity, Including Discontinued Operation	(69)	(621)	1,081
Financing Activities
Dividends paid	(55,489)	(52,602)	(50,814)
Proceeds from reissuance of treasury stock	245	204	245
Purchase of treasury stock	(35,792)	(12,630)	(14,965)
Net cash provided by (used in) financing activities	(91,036)	(65,028)	(65,534)
Net increase (decrease) in cash and cash equivalents	9,248	26,786	22,428
Cash and cash equivalents at January 1	86,909	60,123	37,695
Cash and cash equivalents at December 31	96,157	86,909	60,123
Payments to Acquire Additional Interest in Subsidiaries	$ 38	$ 0	$ 1,120