Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Financial Condition (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Loans	$ 6,137,511	$ 5,726,375
Premises and equipment, net	137,268	80,474
Other assets	97,328	68,409
Total assets	8,308,773	7,828,255
Liabilities and Shareholders' Equity
Subordinated Debentures outstanding, Amount	170,450	170,288
Shareholders' equity	1,055,665	975,389	$ 888,127	$ 749,929
Total liabilities and shareholders’ equity	8,308,773	7,828,255
Parent Company [Member]
Assets
Cash	22,889	21,466
Loans	13	14
Investment in subsidiaries	1,086,844	1,014,685
Investment in unconsolidated subsidiary trusts	2,190	2,192
Investment in Jointly-Owned Company	306	280
Premises and equipment, net	3,801	3,519
Receivables From Subsidiaries	4,750	0
Dividends receivable from subsidiaries	5,097	5,444
Other assets	6,924	5,402
Total assets	1,132,814	1,053,002
Liabilities and Shareholders' Equity
Accrued expenses and other liabilities	4,983	5,446
Subordinated Debentures outstanding, Amount	72,167	72,167
Shareholders' equity	1,055,664	975,389
Total liabilities and shareholders’ equity	$ 1,132,814	$ 1,053,002