Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	$ 325,264	$ 292,257	$ 250,550
Interest expense	(55,402)	(40,035)	(21,770)
Other income	7,268	6,790	7,677
Applicable income tax (provision) benefits	(25,516)	(25,274)	(48,561)
Net Income	105,333	107,498	55,165
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	8	1	1
Dividends from subsidiaries	55,964	81,851	52,586
Interest expense	(3,735)	(3,722)	(3,000)
Other income	6	14	17
Operating expense	(4,525)	(4,047)	(4,767)
Income (loss) before taxes and equity in undistributed earnings of subsidiaries	47,718	74,097	44,837
Applicable income tax (provision) benefits	1,720	1,324	2,557
Income (loss) before equity in undistributed earnings of subsidiaries	49,438	75,421	47,394
Equity in undistributed (loss) earnings of subsidiaries	55,895	32,077	7,771
Net Income	$ 105,333	$ 107,498	$ 55,165