Securities Available for Sale - Amortized cost and fair values of securities available for sale at by contractual maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost
Due in less than one year	$ 35,899
Due in one year through five years	224,042
Due after five years through ten years	72,838
Due after ten years	19,844
Single maturity date, amortized cost	352,623
Amortized Cost	516,905	$ 443,249
Fair Value
Due in less than one year	36,133
Due in one year through five years	231,540
Due after five years through ten years	74,829
Due after ten years	21,094
Single maturity date, fair value	363,596
Fair Value	535,351	449,302
Mortgage-backed securities
Amortized Cost
Without single maturity date, amortized cost	164,282
Amortized Cost	164,282	193,223
Fair Value
Without single maturity date, fair value	171,755
Fair Value	$ 171,755	$ 195,018