Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Amortized Cost [Abstract]
Within one year	$ 36,052
After one year through five years	65,667
After five years through ten years	14,304
After ten years	27,766
Amortized cost, excluding investment securities not due at a single maturity date	143,789
Investment securities not due at a single maturity date, mortgage-backed securities	309,187
Amortized cost	452,976
Fair/Book Value [Abstract]
Within one year	36,053
After one year through five years	65,563
After five years through ten years	14,181
After ten years	27,583
Fair/Book value, excluding investment securities not due at a single maturity date	143,380
Investment securities not due at a single maturity date, mortgage-backed securities	309,462
Fair/Book value	452,842	$ 448,263	$ 341,771
Book Value [Abstract]
Within one year	1,260
After one year through five years	9,463
After five years through ten years	14,010
After ten years	32,548
Book value, excluding investment securities not due at a single maturity date	57,281
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	57,281	58,109	61,535
Fair Value [Abstract]
Within one year	1,262
After one year through five years	9,488
After five years through ten years	14,100
After ten years	32,823
Fair value, excluding investment securities not due at a single maturity date	57,673
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 57,673	$ 58,408	$ 62,892