Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amortized Cost [Abstract]
Within one year	$ 156,840
After one year through five years	17,097
After five years through ten years	1,474
After ten years	13,247
Amortized cost, excluding securities without single maturity date	188,658
Investment securities not due at a single maturity date, mortgage-backed securities	310,981
Amortized cost	499,639	$ 482,757
Fair/Book Value [Abstract]
Within one year	156,751
After one year through five years	17,032
After five years through ten years	1,472
After ten years	13,096
Fair/Book value, excluding securities without single maturity date	188,351
Investment securities not due at a single maturity date, mortgage-backed securities	307,045
Fair/Book value	495,396	481,596
Book Value [Abstract]
Within one year	2,340
After one year through five years	2,161
After five years through ten years	21,167
After ten years	27,898
Book value, excluding securities without single maturity date	53,566
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	53,566	54,460
Fair Value [Abstract]
Within one year	2,342
After one year through five years	2,162
After five years through ten years	21,292
After ten years	27,942
Fair/Book value, excluding securities without single maturity date	53,738
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 53,738	$ 55,236