Condensed Financial Information of First Commonwealth Financial Corporation (parent company only) - Statements of Cash Flow (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Activities
Net Income	$ 59,590	$ 50,143	$ 44,453
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	7,116	7,640	13,721
Other net	3,111	(7,929)	8,253
Net cash provided by operating activities	89,273	72,751	82,137
Investing Activities
Net cash provided by (used in) investing activities	(493,774)	142,433	164,055
Financing Activities
Discount on dividend reinvestment plan purchases	0	0	(65)
Dividends paid	(24,907)	(25,089)	(26,174)
Proceeds from reissuance of treasury stock	216	192	192
Purchase of treasury stock	(864)	(25,383)	(30,956)
Net cash (used in) provided by financing activities	(536,822)	64,596	79,017
Net increase (decrease) in cash and cash equivalents	46,225	(5,086)	(2,901)
Cash and cash equivalents at January 1	69,452	74,538	77,439
Cash and cash equivalents at December 31	115,677	69,452	74,538
Parent [Member]
Operating Activities
Net Income	59,590	50,143	44,453
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	355	493	2,150
Net gain (loss) on sales of assets	0	240	0
Decrease (increase) in prepaid income taxes	7,380	(6,993)	(487)
Undistributed equity in subsidiaries	(8,816)	(4,811)	(127)
Other net	7,759	10,753	13,077
Net cash provided by operating activities	66,268	49,825	59,066
Investing Activities
Net change in loans	(3)	2	3
Payments to Acquire Property and Equipment	0	(54)	47
Proceeds from sale of other assets	(332)	0	(13)
Proceeds from Divestiture of Businesses	27,017	0	0
Payments to Acquire Additional Interest in Subsidiaries	(47,017)	0	0
Net cash provided by (used in) investing activities	(20,329)	(56)	31
Financing Activities
Discount on dividend reinvestment plan purchases	0	0	(65)
Dividends paid	(24,907)	(25,089)	(26,174)
Proceeds from reissuance of treasury stock	216	192	192
Purchase of treasury stock	(864)	(25,383)	(30,956)
Net cash (used in) provided by financing activities	(25,555)	(50,280)	(57,003)
Net increase (decrease) in cash and cash equivalents	20,384	(399)	2,032
Cash and cash equivalents at January 1	10,003	10,402	8,370
Cash and cash equivalents at December 31	$ 30,387	$ 10,003	$ 10,402