Regulatory Restrictions and Capital Adequacy - Schedule of Capital Amount and Ratio Considered to be Capitalized (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Common Equity to Risk Weighted Assets	$ 697,881	$ 623,252
Tier 1 Common Equity to Risk Weighted Assets	11.13%	10.32%
Tier 1 Common Equity Required to be Well Capitalized	$ 407,557	$ 392,620
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	$ 734,246	$ 658,861
Tier 1 Common Equity to Risk Weighted Assets	11.73%	10.94%
Tier 1 Common Equity Required to be Well Capitalized	$ 406,925	$ 391,315
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	$ 918,786	$ 745,473
Tier I Capital to Risk Weighted Assets	767,881	691,993
Tier I Capital to Average Assets	$ 767,881	$ 691,993
Total Capital to Risk Weighted Assets, Ratio	14.65%	12.34%
Tier I Capital to Risk Weighted Assets, Ratio	12.25%	11.46%
Tier I Capital to Average Assets, Ratio	10.28%	9.74%
Actual | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets	$ 885,151	$ 712,341
Tier I Capital to Risk Weighted Assets	734,246	658,861
Tier I Capital to Average Assets	$ 734,246	$ 658,861
Total Capital to Risk Weighted Assets, Ratio	14.14%	11.83%
Tier I Capital to Risk Weighted Assets, Ratio	11.73%	10.94%
Tier I Capital to Average Assets, Ratio	9.84%	9.30%
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	$ 558,728
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	493,771	437,922
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	298,856	284,100
Tier 1 Common Equity Required for Capital Adequacy	$ 399,719	$ 347,317
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio	9.875%	9.25%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio	7.875%	7.25%
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	6.375%	5.75%
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	$ 556,872
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	493,005	436,467
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	298,340	283,344
Tier 1 Common Equity Required for Capital Adequacy	$ 399,100	$ 346,163
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio	9.875%	9.25%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio	7.875%	7.25%
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	6.375%	5.75%
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	$ 658,361	$ 634,232
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	532,959	513,426
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	298,856	284,100
Tier 1 Common Equity Required for Capital Adequacy	$ 438,907	$ 422,821
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 - 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	$ 657,341	$ 632,124
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	532,133	511,720
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	298,340	283,344
Tier 1 Common Equity Required for Capital Adequacy	$ 438,227	$ 421,416
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	$ 501,608	$ 483,224
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	$ 373,570	$ 355,125
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	$ 627,011	$ 604,030
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	$ 500,831	$ 481,619
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	$ 372,926	$ 354,180
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	$ 626,039	$ 602,023
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	10.00%	10.00%