Investments - Investments by contractual maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Within one year, amortized cost	$ 908
Within one year, estimated fair value	925
After one year through five years, amortized cost	14,955
After one year through five years, estimated fair value	15,287
After five years through ten years, amortized cost	39,209
After five years through ten years, estimated fair value	40,256
After ten years, amortized cost	162,040
After ten years, estimated fair value	168,542
Total securities with single maturity date, amortized cost	217,112
Total securities with single maturity date, estimated fair value	225,010
Amortized Cost	520,905
Available-for-sale securities	528,127	$ 547,749
U.S. Government agencies
Schedule of Available-for-sale Securities
Investment securities not due at a single maturity date, amortized cost	63,921
Investment securities not due at a single maturity date, estimated fair value	64,421
Amortized Cost	63,921	69,005
U.S. Government agencies collateralized by residential mortgage obligations
Schedule of Available-for-sale Securities
Investment securities not due at a single maturity date, amortized cost	193,644
Investment securities not due at a single maturity date, estimated fair value	191,903
Amortized Cost	193,644	181,785
Private label mortgage backed securities
Schedule of Available-for-sale Securities
Investment securities not due at a single maturity date, amortized cost	38,728
Investment securities not due at a single maturity date, estimated fair value	39,314
Amortized Cost	38,728	$ 1,807
Other equity securities
Schedule of Available-for-sale Securities
Investment securities not due at a single maturity date, amortized cost	7,500
Investment securities not due at a single maturity date, estimated fair value	$ 7,479