Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Amortized Cost [Abstract]
Within one year	$ 70,271
After one year through five years	20,391
After five years through ten years	797
After ten years	8,237
Amortized cost, excluding investment securities not due at a single maturity date	99,696
Investment securities not due at a single maturity date, mortgage-backed securities	448,904
Amortized cost	548,600	$ 502,693	$ 484,348
Fair/Book Value [Abstract]
Within one year	70,354
After one year through five years	20,901
After five years through ten years	795
After ten years	8,132
Fair value, excluding investment securities not due at a single maturity date	100,182
Investment securities not due at a single maturity date, mortgage-backed securities	469,225
Fair value	569,407	507,386	488,093
Book Value [Abstract]
Within one year	5,497
After one year through five years	4,120
After five years through ten years	25,389
After ten years	34,030
Book value, excluding investment securities not due at a single maturity date	69,036
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	69,036	60,229	60,310
Fair Value [Abstract]
Within one year	5,497
After one year through five years	4,127
After five years through ten years	26,423
After ten years	34,147
Fair value, excluding investment securities not due at a single maturity date	70,194
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 70,194	$ 61,097	$ 61,155