Investment Securities Available-for-Sale - Gross unrealized losses and fair value (Details) $ in Thousands	Sep. 30, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Debt securities as percentage of total investment securities	99.90%
Number of securities | security	63	148
Less than 12 months, estimated fair value	$ 177,650	$ 274,947
Less than 12 months, unrealized losses	766	1,592
12 months or greater, estimated fair value	50,941	142,223
12 months or greater, unrealized losses	674	1,443
Estimated fair value	228,591	417,170
Unrealized losses	$ 1,440	$ 3,035
U. S. agency securities
Number of securities | security	25	36
Less than 12 months, estimated fair value	$ 20,762	$ 75,159
Less than 12 months, unrealized losses	32	439
12 months or greater, estimated fair value	43,607	51,481
12 months or greater, unrealized losses	633	616
Estimated fair value	64,369	126,640
Unrealized losses	$ 665	$ 1,055
Residential mortgage backed securities
Number of securities | security	33	111
Less than 12 months, estimated fair value	$ 139,848	$ 197,794
Less than 12 months, unrealized losses	549	1,148
12 months or greater, estimated fair value	7,334	90,742
12 months or greater, unrealized losses	41	827
Estimated fair value	147,182	288,536
Unrealized losses	$ 590	$ 1,975
Municipal Bonds
Number of securities | security	4	1
Less than 12 months, estimated fair value	$ 14,086	$ 1,994
Less than 12 months, unrealized losses	114	5
12 months or greater, estimated fair value	0	0
12 months or greater, unrealized losses	0	0
Estimated fair value	14,086	1,994
Unrealized losses	$ 114	$ 5
Corporate bonds
Number of securities | security	1
Less than 12 months, estimated fair value	$ 2,954
Less than 12 months, unrealized losses	71
12 months or greater, estimated fair value	0
12 months or greater, unrealized losses	0
Estimated fair value	2,954
Unrealized losses	$ 71