Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Amortized Cost [Abstract]
Within one year	$ 156,785
After one year through five years	16,814
After five years through ten years	1,387
After ten years	12,405
Amortized cost, excluding investment securities not due at a single maturity date	187,391
Investment securities not due at a single maturity date, mortgage-backed securities	303,895
Amortized cost	491,286	$ 499,639	$ 502,262
Fair/Book Value [Abstract]
Within one year	156,762
After one year through five years	16,856
After five years through ten years	1,386
After ten years	12,265
Fair/Book value, excluding investment securities not due at a single maturity date	187,269
Investment securities not due at a single maturity date, mortgage-backed securities	304,020
Fair/Book value	491,289	495,396	495,814
Book Value [Abstract]
Within one year	760
After one year through five years	3,329
After five years through ten years	24,797
After ten years	25,526
Book value, excluding investment securities not due at a single maturity date	54,412
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	54,412	53,566	53,527
Fair Value [Abstract]
Within one year	761
After one year through five years	3,330
After five years through ten years	25,189
After ten years	25,702
Fair value, excluding investment securities not due at a single maturity date	54,982
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 54,982	$ 53,738	$ 53,745