Condensed Financial Information of First Commonwealth Financial Corporation (parent company only) - Statements of Cash Flow (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating Activities
Net Income	$ 138,257	$ 73,447	$ 105,333
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	11,442	11,653	10,370
Other net	(5,874)	5,349	(14,310)
Net cash provided by operating activities	165,046	105,699	107,632
Investing Activities
Net cash (used in) provided by investing activities	(565,305)	(483,138)	122,942
Financing Activities
Dividends paid	(43,611)	(42,982)	(39,394)
Proceeds from reissuance of treasury stock	222	222	211
Purchase of treasury stock	(31,301)	(20,905)	(6,259)
Net cash provided by (used in) financing activities	439,050	612,164	(207,665)
Net increase in cash and cash equivalents	38,791	234,725	22,909
Parent Company [Member]
Operating Activities
Net Income	138,257	73,447	105,333
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	369	383	340
Net gains on sales of assets	228	0	(2)
Decrease (increase) in prepaid income taxes	317	(317)	629
Undistributed equity in subsidiaries	(72,330)	(17,996)	(55,895)
Other net	(14,830)	19,705	(2,957)
Net cash provided by operating activities	52,011	75,222	47,448
Investing Activities
Net change in loans	2	1	1
Payments to Acquire Property and Equipment	(101)	(20)	(586)
Proceeds from sale of other assets	0	0	2
Net cash (used in) provided by investing activities	(99)	(19)	(583)
Financing Activities
Dividends paid	(43,611)	(42,982)	(39,394)
Proceeds from reissuance of treasury stock	222	222	211
Purchase of treasury stock	(31,301)	(20,905)	(6,259)
Net cash provided by (used in) financing activities	(74,690)	(63,665)	(45,442)
Net increase in cash and cash equivalents	(22,778)	11,538	1,423
Cash and cash equivalents at January 1	34,427	22,889	21,466
Cash and cash equivalents at December 31	$ 11,649	$ 34,427	$ 22,889