Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Financial Condition (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Loans	$ 6,659,874	$ 6,137,511
Premises and equipment, net	125,517	137,268
Other assets	143,415	97,328
Total assets	9,068,104	8,308,773
Liabilities and Shareholders' Equity
Subordinated Debentures outstanding, Amount	170,612	170,450
Shareholders' equity	1,068,617	1,055,665	$ 975,389	$ 888,127
Total liabilities and shareholders’ equity	9,068,104	8,308,773
Parent Company [Member]
Assets
Cash	34,427	22,889
Loans	11	13
Investment in subsidiaries	1,108,801	1,086,844
Investment in unconsolidated subsidiary trusts	2,182	2,190
Investment in Jointly-Owned Company	338	306
Premises and equipment, net	3,476	3,801
Receivables From Subsidiaries	0	4,750
Dividends receivable from subsidiaries	3,038	5,097
Other assets	7,307	6,924
Total assets	1,159,580	1,132,814
Liabilities and Shareholders' Equity
Accrued expenses and other liabilities	18,796	4,983
Subordinated Debentures outstanding, Amount	72,167	72,167
Shareholders' equity	1,068,617	1,055,664
Total liabilities and shareholders’ equity	$ 1,159,580	$ 1,132,814