Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Operations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	$ 202,181	$ 206,358	$ 219,075
Interest expense	(18,501)	(21,707)	(30,146)
Other income	7,445	12,060	14,365
Applicable income tax (provision) benefits	(17,680)	(15,281)	(14,658)
Net Income	44,453	41,482	41,954
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Interest and dividends	1	1	1
Dividends from subsidiaries	49,207	65,140	64,342
Interest expense	(2,335)	(3,128)	(5,711)
Other income	1,251	2,653	12,581
Operating expense	(6,766)	(8,820)	(19,061)
Income (loss) before taxes and equity in undistributed earnings of subsidiaries	41,358	55,846	52,152
Applicable income tax (provision) benefits	2,968	3,384	4,364
Income (loss) before equity in undistributed earnings of subsidiaries	44,326	59,230	56,516
Equity in undistributed (loss) earnings of subsidiaries	127	(17,748)	(14,562)
Net Income	$ 44,453	$ 41,482	$ 41,954