Regulatory Restrictions and Capital Adequacy - Schedule of Capital Amount and Ratio Considered to be Capitalized (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Common Equity to Risk Weighted Assets	$ 730,526	$ 697,881
Tier 1 Common Equity to Risk Weighted Assets	10.91%	11.13%
Tier 1 Common Equity Required to be Well Capitalized	$ 435,420	$ 407,557
Tier 1 Common Equity Required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Common Equity to Risk Weighted Assets	$ 759,398	$ 734,246
Tier 1 Common Equity to Risk Weighted Assets	11.36%	11.73%
Tier 1 Common Equity Required to be Well Capitalized	$ 434,575	$ 406,925
Tier 1 Common Equity Required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Actual | First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets	$ 954,991	$ 918,786
Tier I Capital to Risk Weighted Assets	800,526	767,881
Tier I Capital to Average Assets	$ 800,526	$ 767,881
Total Capital to Risk Weighted Assets, Ratio	14.26%	14.65%
Tier I Capital to Risk Weighted Assets, Ratio	11.95%	12.25%
Tier I Capital to Average Assets, Ratio	10.17%	10.28%
Actual | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets	$ 913,863	$ 885,151
Tier I Capital to Risk Weighted Assets	759,398	734,246
Tier I Capital to Average Assets	$ 759,398	$ 734,246
Total Capital to Risk Weighted Assets, Ratio	13.67%	14.14%
Tier I Capital to Risk Weighted Assets, Ratio	11.36%	11.73%
Tier I Capital to Average Assets, Ratio	9.66%	9.84%
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		$ 619,173
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount		493,771
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount		298,856
Tier 1 Common Equity Required for Capital Adequacy		$ 399,719
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio		9.88%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio		7.88%
Tier I Capital to Average Assets, Regulatory Minimum Ratio		4.00%
Tier 1 Common Equity Required for Capital Adequacy to Risk Weighted Assets		6.38%
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		$ 618,213
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount		493,005
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount		298,340
Tier 1 Common Equity Required for Capital Adequacy		$ 399,100
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio		9.88%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio		7.88%
Tier I Capital to Average Assets, Regulatory Minimum Ratio		4.00%
Tier 1 Common Equity Required for Capital Adequacy to Risk Weighted Assets		6.38%
Minimum Capital Required | First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	$ 703,370	$ 658,361
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	569,395	532,959
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	314,963	298,856
Tier 1 Common Equity Required for Capital Adequacy	$ 468,914	$ 438,907
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio	10.50%	10.50%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio	8.50%	8.50%
Tier I Capital to Average Assets, Regulatory Minimum Ratio	4.00%	4.00%
Tier 1 Common Equity Required for Capital Adequacy to Risk Weighted Assets	7.00%	7.00%
Minimum Capital Required | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	$ 702,006	$ 657,341
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	568,291	532,133
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	314,338	298,340
Tier 1 Common Equity Required for Capital Adequacy	$ 468,004	$ 438,227
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio	10.50%	10.50%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio	8.50%	8.50%
Tier I Capital to Average Assets, Regulatory Minimum Ratio	4.00%	4.00%
Tier 1 Common Equity Required for Capital Adequacy to Risk Weighted Assets	7.00%	7.00%
Required to be Considered Well Capitalized | First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier One Risk Based Capital Required to be Well Capitalized	$ 535,901	$ 501,608
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	8.00%	8.00%
Tier One Leverage Capital Required to be Well Capitalized	$ 393,704	$ 373,570
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	5.00%	5.00%
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	$ 669,877	$ 627,011
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	10.00%	10.00%
Required to be Considered Well Capitalized | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier One Risk Based Capital Required to be Well Capitalized	$ 534,862	$ 500,831
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	8.00%	8.00%
Tier One Leverage Capital Required to be Well Capitalized	$ 392,922	$ 372,926
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	5.00%	5.00%
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	$ 668,577	$ 626,039
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	10.00%	10.00%