Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost [Abstract]
Within one year	$ 113,065
After one year through five years	30,207
After five years through ten years	13,044
After ten years	23,939
Amortized cost, excluding securities without single maturity date	180,255
Investment securities not due at a single maturity date, mortgage-backed securities	302,502
Amortized cost	482,757
Fair/Book Value [Abstract]
Within one year	112,989
After one year through five years	29,979
After five years through ten years	12,922
After ten years	23,792
Fair/Book value, excluding securities without single maturity date	179,682
Investment securities not due at a single maturity date, mortgage-backed securities	301,914
Fair/Book value	481,596	$ 448,263
Book Value [Abstract]
Within one year	1,760
After one year through five years	8,659
After five years through ten years	14,155
After ten years	29,886
Book value, excluding securities without single maturity date	54,460
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	54,460	58,109
Fair Value [Abstract]
Within one year	1,769
After one year through five years	8,664
After five years through ten years	14,347
After ten years	30,456
Fair/Book value, excluding securities without single maturity date	55,236
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 55,236	$ 58,408