Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Financial Condition (Detail) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Assets
Financing receivable, before allowance for credit loss	$ 9,508,039	$ 8,983,754
Premises and equipment, net	114,283	116,108
Other assets	188,384	220,536
Total assets	12,343,036	11,584,936
Liabilities and Shareholders' Equity
Subordinated Debentures outstanding, Amount	128,466	128,305
Shareholders' equity	1,554,376	1,405,165	$ 1,314,274	$ 1,052,074
Total liabilities and shareholders’ equity	12,343,036	11,584,936
Parent Company [Member]
Assets
Cash	96,157	86,909
Financing receivable, before allowance for credit loss	0	3
Investment in subsidiaries	1,518,147	1,378,052
Investment in unconsolidated subsidiary trusts	2,200	2,204
Investment in Jointly-Owned Company	377	438
Premises and equipment, net	3,022	3,184
Dividends receivable from subsidiaries	8,132	6,772
Other assets	17,872	17,546
Total assets	1,645,909	1,495,108
Liabilities and Shareholders' Equity
Accrued expenses and other liabilities	12,547	11,048
Subordinated Debentures outstanding, Amount	78,986	78,895
Shareholders' equity	1,554,376	1,405,165
Total liabilities and shareholders’ equity	1,645,909	1,495,108
Receivables From Subsidiaries	$ 2	$ 0