Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	$ 301,209	$ 325,264	$ 292,257
Interest expense	(32,938)	(55,402)	(40,035)
Other income	7,892	7,268	6,790
Applicable income tax (provision) benefits	(16,756)	(25,516)	(25,274)
Net Income	73,447	105,333	107,498
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	8	8	1
Dividends from subsidiaries	61,708	55,964	81,851
Interest expense	(3,229)	(3,735)	(3,722)
Other income	3	6	14
Operating expense	(4,687)	(4,525)	(4,047)
Income (loss) before taxes and equity in undistributed earnings of subsidiaries	53,803	47,718	74,097
Applicable income tax (provision) benefits	1,648	1,720	1,324
Income (loss) before equity in undistributed earnings of subsidiaries	55,451	49,438	75,421
Equity in undistributed (loss) earnings of subsidiaries	17,996	55,895	32,077
Net Income	$ 73,447	$ 105,333	$ 107,498