Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Amortized Cost [Abstract]
Within one year	$ 85,403
After one year through five years	25,426
After five years through ten years	10,198
After ten years	7,763
Amortized cost, excluding investment securities not due at a single maturity date	128,790
Investment securities not due at a single maturity date, mortgage-backed securities	421,263
Amortized cost	550,053	$ 502,693	$ 488,503
Fair Value [Abstract]
Within one year	85,463
After one year through five years	25,963
After five years through ten years	10,169
After ten years	7,667
Fair value, excluding investment securities not due at a single maturity date	129,262
Investment securities not due at a single maturity date, mortgage-backed securities	439,274
Fair value	568,536	507,386	492,930
Book Value [Abstract]
Within one year	7,898
After one year through five years	4,816
After five years through ten years	24,567
After ten years	32,632
Book value, excluding investment securities not due at a single maturity date	69,913
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	69,913	60,229	60,354
Fair Value [Abstract]
Within one year	7,898
After one year through five years	4,863
After five years through ten years	25,560
After ten years	32,734
Fair value, excluding investment securities not due at a single maturity date	71,055
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 71,055	$ 61,097	$ 61,293