Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	$ 293,838	$ 301,209	$ 325,264
Interest expense	(15,297)	(32,938)	(55,402)
Other income	8,185	7,892	7,268
Applicable income tax (provision) benefits	(34,560)	(16,756)	(25,516)
Net Income	138,257	73,447	105,333
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	4	8	8
Dividends from subsidiaries	72,202	61,708	55,964
Interest expense	(3,205)	(3,229)	(3,735)
Other income	0	3	6
Operating expense	(4,721)	(4,687)	(4,525)
Income (loss) before taxes and equity in undistributed earnings of subsidiaries	64,280	53,803	47,718
Applicable income tax (provision) benefits	1,646	1,648	1,720
Income (loss) before equity in undistributed earnings of subsidiaries	65,926	55,451	49,438
Equity in undistributed (loss) earnings of subsidiaries	72,331	17,996	55,895
Net Income	$ 138,257	$ 73,447	$ 105,333