Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Financial Condition (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Loans	$ 4,405,257	$ 4,229,608
Premises and equipment, net	64,989	67,940
Other assets	110,777	138,698
Total assets	6,360,285	6,214,861
Liabilities and Shareholders' Equity
Subordinated debentures payable	72,167	72,167
Shareholders' equity	716,145	711,697	746,007	758,543
Total liabilities and shareholders’ equity	6,360,285	6,214,861
Parent [Member]
Assets
Cash	10,402	8,370
Loans	24	27
Investment in subsidiaries	712,610	696,438
Investment in unconsolidated subsidiary trusts	2,182	2,182
Investment in Jointly-Owned Company	8,749	8,559
Premises and equipment, net	4,287	6,376
Recievable from subsidiaries	0	267
Dividends receivable from subsidiaries	0	1,319
Other assets	50,988	62,633
Total assets	789,242	786,171
Liabilities and Shareholders' Equity
Accrued expenses and other liabilities	930	2,307
Subordinated debentures payable	72,167	72,167
Shareholders' equity	716,145	711,697
Total liabilities and shareholders’ equity	$ 789,242	$ 786,171