Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	$ 329,953	$ 293,838	$ 301,209
Interest expense	(17,732)	(15,297)	(32,938)
Other income	10,263	8,185	7,892
Applicable income tax (provision) benefits	(32,004)	(34,560)	(16,756)
Net Income	128,181	138,257	73,447
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	78	4	8
Dividends from subsidiaries	92,082	72,202	61,708
Interest expense	(3,245)	(3,205)	(3,229)
Other income	112	0	3
Operating expense	(4,747)	(4,721)	(4,687)
Income (loss) before taxes and equity in undistributed earnings of subsidiaries	84,280	64,280	53,803
Applicable income tax (provision) benefits	1,625	1,646	1,648
Income (loss) before equity in undistributed earnings of subsidiaries	85,905	65,926	55,451
Equity in undistributed (loss) earnings of subsidiaries	42,276	72,331	17,996
Net Income	$ 128,181	$ 138,257	$ 73,447