Securities Available for Sale - Amortized cost and fair values of securities available for sale at by contractual maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Amortized Cost
Due in less than one year	$ 16,942
Due in one year through five years	235,471
Due after five years through ten years	35,066
Due after ten years	10,424
Single maturity date, amortized cost	297,903
Fair Value
Due in less than one year	16,982
Due in one year through five years	237,604
Due after five years through ten years	35,929
Due after ten years	11,392
Single maturity date, fair value	301,907
Amortized Cost	501,478	$ 443,249
Debt Securities, Available-for-sale	511,860	449,302
Mortgage-backed securities
Amortized Cost
Amortized cost	203,575
Fair Value
Fair value	209,953
Amortized Cost	203,575	193,223
Debt Securities, Available-for-sale	$ 209,953	$ 195,018