Condensed Financial Information of First Commonwealth Financial Corporation (parent company only) - Statements of Cash Flow (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities
Net Income	$ 105,333	$ 107,498	$ 55,165
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	10,370	8,046	8,997
Other net	(14,310)	11,695	10,997
Net cash provided by operating activities	107,632	135,263	88,305
Investing Activities
Net cash provided by (used in) investing activities	(122,942)	347,748	52,797
Financing Activities
Dividends paid	(39,394)	(34,849)	(30,513)
Proceeds from reissuance of treasury stock	211	208	228
Purchase of treasury stock	(6,259)	(26,189)	(1,458)
Net cash (used in) provided by financing activities	(207,665)	204,140	(43,893)
Net increase (decrease) in cash and cash equivalents	22,909	(8,345)	(8,385)
Cash and cash equivalents at January 1	98,947	107,292	115,677
Cash and cash equivalents at December 31	121,856	98,947	107,292
Parent Company [Member]
Operating Activities
Net Income	105,333	107,498	55,165
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	340	320	322
Net gains on sales of assets	(2)	(7)	(3)
Decrease (increase) in prepaid income taxes	629	37	(550)
Undistributed equity in subsidiaries	(55,895)	(32,077)	(7,771)
Proceeds from Equity Method Investment, Distribution	0	9,000	0
Other net	(2,957)	(1,628)	8,767
Net cash provided by operating activities	47,448	83,143	55,930
Investing Activities
Net change in loans	1	3	2
Payments to Acquire Property and Equipment	(586)	(87)	(207)
Proceeds from sale of other assets	2	7	3
Payments to Acquire Interest in Subsidiaries and Affiliates	0	0	(250)
Payments to Acquire Additional Interest in Subsidiaries	0	(17,202)	(37,690)
Net cash provided by (used in) investing activities	(583)	17,279	38,142
Financing Activities
Dividends paid	(39,394)	(34,849)	(30,513)
Proceeds from reissuance of treasury stock	211	208	228
Purchase of treasury stock	(6,259)	(26,189)	(1,458)
Net cash (used in) provided by financing activities	(45,442)	(60,830)	(31,743)
Net increase (decrease) in cash and cash equivalents	1,423	5,034	(13,955)
Cash and cash equivalents at January 1	21,466	16,432	30,387
Cash and cash equivalents at December 31	$ 22,889	$ 21,466	$ 16,432