Investments - Investments by contractual maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities
Within one year, amortized cost	$ 0
Within one year, estimated fair value	0
After one year through five years, amortized cost	13,635
After one year through five years, estimated fair value	14,185
After five years through ten years, amortized cost	38,100
After five years through ten years, estimated fair value	39,642
After ten years, amortized cost	152,507
After ten years, estimated fair value	160,930
Total securities with single maturity date, amortized cost	204,242
Total securities with single maturity date, estimated fair value	214,757
Debt securities, amortized cost	488,425
Available-for-sale investment securities	501,978	$ 477,554
U.S. Government agencies
Schedule of Available-for-sale Securities
Investment securities not due at a single maturity date, amortized cost	51,194
Investment securities not due at a single maturity date, estimated fair value	51,558
Debt securities, amortized cost	51,194	52,803
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	223,234
Investment securities not due at a single maturity date, estimated fair value	224,641
Debt securities, amortized cost	223,234	225,636
Private label residential mortgage backed securities
Schedule of Available-for-sale Securities
Investment securities not due at a single maturity date, amortized cost	2,255
Investment securities not due at a single maturity date, estimated fair value	3,374
Debt securities, amortized cost	2,255	$ 2,356
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	$ 7,648