Condensed Financial Information of First Commonwealth Financial Corporation (parent company only) - Statements of Cash Flow (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities
Net Income	$ 73,447	$ 105,333	$ 107,498
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	11,653	10,370	8,046
Other net	5,349	(14,310)	11,695
Net cash provided by operating activities	105,699	107,632	135,263
Investing Activities
Net cash (used in) provided by investing activities	(483,138)	122,942	(347,748)
Financing Activities
Dividends paid	(42,982)	(39,394)	(34,849)
Proceeds from reissuance of treasury stock	222	211	208
Purchase of treasury stock	(20,905)	(6,259)	(26,189)
Net cash provided by (used in) financing activities	612,164	(207,665)	204,140
Parent Company [Member]
Operating Activities
Net Income	73,447	105,333	107,498
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	383	340	320
Net gains on sales of assets	0	(2)	(7)
Decrease (increase) in prepaid income taxes	(317)	629	37
Undistributed equity in subsidiaries	(17,996)	(55,895)	(32,077)
Proceeds from Equity Method Investment, Distribution	0	0	9,000
Other net	19,705	(2,957)	(1,628)
Net cash provided by operating activities	75,222	47,448	83,143
Investing Activities
Net change in loans	1	1	3
Payments to Acquire Property and Equipment	(20)	(586)	(87)
Proceeds from sale of other assets	0	2	7
Payments to Acquire Additional Interest in Subsidiaries	0	0	(17,202)
Net cash (used in) provided by investing activities	(19)	(583)	(17,279)
Financing Activities
Dividends paid	(42,982)	(39,394)	(34,849)
Proceeds from reissuance of treasury stock	222	211	208
Purchase of treasury stock	(20,905)	(6,259)	(26,189)
Net cash provided by (used in) financing activities	(63,665)	(45,442)	(60,830)
Net (decrease) increase in cash and cash equivalents	11,538	1,423	5,034
Cash and cash equivalents at January 1	22,889	21,466	16,432
Cash and cash equivalents at December 31	$ 34,427	$ 22,889	$ 21,466