Securities Available for Sale - Amortized cost and fair values of securities available for sale at by contractual maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Amortized Cost
Due in less than one year	$ 28,148
Due in one year through five years	217,289
Due after five years through ten years	42,898
Due after ten years	15,452
Single maturity date, amortized cost	303,787
Amortized Cost	492,606	$ 443,249
Fair Value
Due in less than one year	28,294
Due in one year through five years	224,722
Due after five years through ten years	44,561
Due after ten years	16,336
Single maturity date, fair value	313,913
Fair Value	510,809	449,302
Mortgage-backed securities
Amortized Cost
Without single maturity date, amortized cost	188,819
Amortized Cost	188,819	193,223
Fair Value
Without single maturity date, fair value	196,896
Fair Value	$ 196,896	$ 195,018