Securities Available for Sale - Amortized cost and fair values of securities available for sale at by contractual maturity (Details 2) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in less than one year	$ 17,307
Amortized Cost, Due in one year through five years	128,152
Amortized Cost, Due after five years through ten years	102,246
Amortized Cost, Due after ten years	7,267
Available for sale, Single maturity date, Amortized Cost	254,972
Amortized Cost, Securities available for sale	410,012	$ 408,093
Fair Value, Due in less than one year	17,278
Fair Value, Due in one year through five years	126,324
Fair Value, Due after five years through ten years	98,947
Fair Value, Due after ten years	7,458
Available for sale, Single maturity date, Fair value	250,007
Fair Value, Securities available for sale	401,130	405,153
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Securities available for sale	155,040	157,705
Fair Value, Securities available for sale	$ 151,123	$ 155,532