Condensed Financial Information of First Commonwealth Financial Corporation (parent company only) - Statements of Cash Flow (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Activities
Net Income	$ 44,453	$ 41,482	$ 41,954
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	13,721	11,090	7,912
Other net	8,253	(2,426)	(4,079)
Net cash provided by operating activities	82,137	85,734	78,724
Investing Activities
Purchases of premises and equipment	(10,980)	(9,635)	(10,182)
Proceeds from sale of other assets	12,882	12,713	17,660
Net cash used in investing activities	(164,055)	(307,280)	(171,382)
Financing Activities
Repayments of subordinated debentures	0	(34,702)	0
Discount on dividend reinvestment plan purchases	(65)	(112)	(92)
Dividends paid	(26,174)	(22,344)	(18,759)
Proceeds from reissuance of treasury stock	192	176	1,028
Purchase of treasury stock	(30,956)	(33,439)	(36,242)
Stock option tax benefit	0	0	1
Net cash provided by financing activities	79,017	196,003	117,162
Net (decrease) increase in cash and cash equivalents	(2,901)	(25,543)	24,504
Cash and cash equivalents at January 1	77,439	102,982	78,478
Cash and cash equivalents at December 31	74,538	77,439	102,982
Parent [Member]
Operating Activities
Net Income	44,453	41,482	41,954
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	2,150	3,030	3,719
Net gain (loss) on sales of assets	0	17	(107)
(Increase) decrease in prepaid income taxes	(487)	3,044	(3,044)
Undistributed equity in subsidiaries	(127)	17,748	14,562
Other net	13,077	12,964	8,789
Net cash provided by operating activities	59,066	78,285	65,873
Investing Activities
Net change in loans	3	4	4
Purchases of premises and equipment	(47)	(123)	(3,005)
Proceeds from sale of other assets	13	1,132	4,309
Net cash used in investing activities	(31)	1,013	1,308
Financing Activities
Repayments of subordinated debentures	0	(34,702)	0
Discount on dividend reinvestment plan purchases	(65)	(112)	(92)
Dividends paid	(26,174)	(22,344)	(18,759)
Proceeds from reissuance of treasury stock	192	176	1,028
Purchase of treasury stock	(30,956)	(33,439)	(36,242)
Stock option tax benefit	0	0	1
Net cash provided by financing activities	(57,003)	(90,421)	(54,064)
Net (decrease) increase in cash and cash equivalents	2,032	(11,123)	13,117
Cash and cash equivalents at January 1	8,370	19,493	6,376
Cash and cash equivalents at December 31	$ 10,402	$ 8,370	$ 19,493