Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Amortized Cost [Abstract]
Within one year	$ 111,092
After one year through five years	30,477
After five years through ten years	13,429
After ten years	25,492
Amortized cost, excluding investment securities not due at a single maturity date	180,490
Investment securities not due at a single maturity date, mortgage-backed securities	274,877
Amortized cost	455,367
Fair/Book Value [Abstract]
Within one year	111,024
After one year through five years	30,394
After five years through ten years	13,408
After ten years	25,454
Fair/Book value, excluding investment securities not due at a single maturity date	180,280
Investment securities not due at a single maturity date, mortgage-backed securities	276,043
Fair/Book value	456,323	$ 448,263	$ 358,857
Book Value [Abstract]
Within one year	1,985
After one year through five years	9,221
After five years through ten years	13,694
After ten years	30,642
Book value, excluding investment securities not due at a single maturity date	55,542
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	55,542	58,109	58,905
Fair Value [Abstract]
Within one year	1,989
After one year through five years	9,238
After five years through ten years	13,887
After ten years	31,177
Fair value, excluding investment securities not due at a single maturity date	56,291
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 56,291	$ 58,408	$ 60,454