Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Amortized Cost [Abstract]
Within one year	$ 128,110
After one year through five years	18,748
After five years through ten years	1,583
After ten years	15,898
Amortized cost, excluding investment securities not due at a single maturity date	164,339
Investment securities not due at a single maturity date, mortgage-backed securities	293,473
Amortized cost	457,812	$ 482,757	$ 461,231
Fair/Book Value [Abstract]
Within one year	127,874
After one year through five years	18,582
After five years through ten years	1,580
After ten years	15,724
Fair/Book value, excluding investment securities not due at a single maturity date	163,760
Investment securities not due at a single maturity date, mortgage-backed securities	286,414
Fair/Book value	450,174	481,596	461,867
Book Value [Abstract]
Within one year	3,613
After one year through five years	5,750
After five years through ten years	19,192
After ten years	23,655
Book value, excluding investment securities not due at a single maturity date	52,210
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	52,210	54,460	56,381
Fair Value [Abstract]
Within one year	3,618
After one year through five years	5,753
After five years through ten years	19,258
After ten years	23,729
Fair value, excluding investment securities not due at a single maturity date	52,358
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 52,358	$ 55,236	$ 57,127