Subordinated Debentures Schedule of Subordinated Debentures Outstanding (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Subordinated debentures	$ 170,249	$ 72,167
First Commonwealth Bank 2028 [Member]
Debt Instrument [Line Items]
Subordinated debentures	49,109
First Commonwealth Bank 2033 [Member]
Debt Instrument [Line Items]
Subordinated debentures	48,973
First Commonwealth Capital Trust II [Member]
Debt Instrument [Line Items]
Subordinated debentures	30,929	30,929
First Commonwealth Capital Trust III [Member]
Debt Instrument [Line Items]
Subordinated debentures	$ 41,238	$ 41,238