Regulatory Restrictions and Capital Adequacy - Schedule of Capital Amount and Ratio Considered to be Capitalized (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Common Equity to Risk Weighted Assets	$ 556,939
Tier 1 Common Equity to Risk Weighted Assets	10.00%
Tier 1 Common Equity Required to be Well Capitalized	$ 362,091
Tier 1 Common Equity Required to be Well Capitalized to Risk Weighted Assets	6.50%
First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Common Equity to Risk Weighted Assets	$ 535,814
Tier 1 Common Equity to Risk Weighted Assets	9.62%
Tier 1 Common Equity Required to be Well Capitalized	$ 362,084
Tier 1 Common Equity Required to be Well Capitalized to Risk Weighted Assets	6.50%
Actual | First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets	$ 682,199	$ 662,733
Tier I Capital to Risk Weighted Assets	626,939	607,602
Tier I Capital to Average Assets	$ 626,939	$ 607,602
Total Capital to Risk Weighted Assets, Ratio	12.25%	12.79%
Tier I Capital to Risk Weighted Assets, Ratio	11.25%	11.73%
Tier I Capital to Average Assets, Ratio	9.85%	9.85%
Actual | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets	$ 656,244	$ 647,500
Tier I Capital to Risk Weighted Assets	600,984	592,369
Tier I Capital to Average Assets	$ 600,984	$ 592,369
Total Capital to Risk Weighted Assets, Ratio	11.78%	12.48%
Tier I Capital to Risk Weighted Assets, Ratio	10.79%	11.41%
Tier I Capital to Average Assets, Ratio	9.46%	9.66%
Minimum Capital Required - Basel III Phase-In Schedule | First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	$ 445,650
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	334,237
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	254,631
Tier 1 Common Equity Required for Capital Adequacy	$ 250,678
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio	8.00%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio	6.00%
Tier I Capital to Average Assets, Regulatory Minimum Ratio	4.00%
Tier 1 Common Equity Required for Capital Adequacy to Risk Weighted Assets	4.50%
Minimum Capital Required - Basel III Phase-In Schedule | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	$ 445,642
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	334,231
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	254,095
Tier 1 Common Equity Required for Capital Adequacy	$ 250,673
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio	8.00%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio	6.00%
Tier I Capital to Average Assets, Regulatory Minimum Ratio	4.00%
Tier 1 Common Equity Required for Capital Adequacy to Risk Weighted Assets	4.50%
Minimum Capital Required - Basel III Fully Phased-In | First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	$ 584,915
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	473,503
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	254,631
Tier 1 Common Equity Required for Capital Adequacy	$ 389,944
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio	10.50%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio	8.50%
Tier I Capital to Average Assets, Regulatory Minimum Ratio	4.00%
Tier 1 Common Equity Required for Capital Adequacy to Risk Weighted Assets	7.00%
Minimum Capital Required - Basel III Fully Phased-In | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	$ 584,905
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	473,494
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	254,095
Tier 1 Common Equity Required for Capital Adequacy	$ 389,937
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio	10.50%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio	8.50%
Tier I Capital to Average Assets, Regulatory Minimum Ratio	4.00%
Tier 1 Common Equity Required for Capital Adequacy to Risk Weighted Assets	7.00%
Regulatory Minimum | First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount		$ 414,460
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount		207,230
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount		$ 246,738
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio		8.00%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio		4.00%
Tier I Capital to Average Assets, Regulatory Minimum Ratio		4.00%
Regulatory Minimum | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount		$ 415,217
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount		207,608
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount		$ 245,276
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio		8.00%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio		4.00%
Tier I Capital to Average Assets, Regulatory Minimum Ratio		4.00%
Required to be Considered Well Capitalized | First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	$ 557,062
Tier I Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines, Capital Amount	445,650
Tier I Capital to Average Assets, Well Capitalized Regulatory Guidelines, Capital Amount	$ 318,289
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	10.00%
Tier I Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines Ratio	8.00%
Tier I Capital to Average Assets, Well Capitalized Regulatory Guidelines Ratio	5.00%
Required to be Considered Well Capitalized | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	$ 557,052	$ 519,021
Tier I Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines, Capital Amount	445,642	311,413
Tier I Capital to Average Assets, Well Capitalized Regulatory Guidelines, Capital Amount	$ 317,618	$ 306,595
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	10.00%	10.00%
Tier I Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines Ratio	8.00%	6.00%
Tier I Capital to Average Assets, Well Capitalized Regulatory Guidelines Ratio	5.00%	5.00%