Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	$ 217,614	$ 204,071	$ 202,181
Interest expense	(18,579)	(15,595)	(18,501)
Other income	6,372	7,041	7,279
Applicable income tax (provision) benefits	(25,639)	(20,836)	(17,680)
Net Income	59,590	50,143	44,453
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	1	1	1
Dividends from subsidiaries	55,510	49,917	49,207
Interest expense	(2,635)	(2,357)	(2,335)
Other income	83	232	1,251
Operating expense	(4,700)	(4,989)	(6,766)
Income (loss) before taxes and equity in undistributed earnings of subsidiaries	48,259	42,804	41,358
Applicable income tax (provision) benefits	2,515	2,528	2,968
Income (loss) before equity in undistributed earnings of subsidiaries	50,774	45,332	44,326
Equity in undistributed (loss) earnings of subsidiaries	8,816	4,811	127
Net Income	$ 59,590	$ 50,143	$ 44,453