Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Amortized Cost [Abstract]
Within one year	$ 5,574
After one year through five years	15,092
After five years through ten years	829
After ten years	8,842
Amortized cost, excluding investment securities not due at a single maturity date	30,337
Investment securities not due at a single maturity date, mortgage-backed securities	482,439
Amortized cost	512,776	$ 502,693	$ 491,286
Fair Value [Abstract]
Within one year	5,574
After one year through five years	15,713
After five years through ten years	829
After ten years	8,754
Fair value, excluding investment securities not due at a single maturity date	30,870
Investment securities not due at a single maturity date, mortgage-backed securities	501,376
Fair value	532,246	507,386	491,289
Book Value [Abstract]
Within one year	3,980
After one year through five years	4,894
After five years through ten years	24,064
After ten years	25,763
Book value, excluding investment securities not due at a single maturity date	58,701
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	58,701	60,229	54,412
Fair Value [Abstract]
Within one year	3,980
After one year through five years	4,899
After five years through ten years	24,783
After ten years	25,841
Fair value, excluding investment securities not due at a single maturity date	59,503
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 59,503	$ 61,097	$ 54,982