Investments - Investments by contractual maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Within one year, amortized cost	$ 1,532
Within one year, estimated fair value	1,557
After one year through five years, amortized cost	6,294
After one year through five years, estimated fair value	6,523
After five years through ten years, amortized cost	31,333
After five years through ten years, estimated fair value	32,336
After ten years, amortized cost	165,682
After ten years, estimated fair value	172,569
Total securities with single maturity date, amortized cost	204,841
Total securities with single maturity date, estimated fair value	212,985
Amortized Cost	507,477
Available-for-sale securities	515,077	$ 547,749
U.S. Government agencies
Schedule of Available-for-sale Securities
Investment securities not due at a single maturity date, amortized cost	61,253
Investment securities not due at a single maturity date, estimated fair value	61,804
Amortized Cost	61,253	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	190,028
Investment securities not due at a single maturity date, estimated fair value	188,349
Amortized Cost	190,028	181,785
Private label mortgage backed securities
Schedule of Available-for-sale Securities
Investment securities not due at a single maturity date, amortized cost	43,855
Investment securities not due at a single maturity date, estimated fair value	44,453
Amortized Cost	43,855	$ 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	$ 7,486