Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Amortized Cost [Abstract]
Within one year	$ 156,111
After one year through five years	15,601
After five years through ten years	1,566
After ten years	14,593
Amortized cost, excluding investment securities not due at a single maturity date	187,871
Investment securities not due at a single maturity date, mortgage-backed securities	279,757
Amortized cost	467,628	$ 482,757	$ 455,367
Fair/Book Value [Abstract]
Within one year	155,934
After one year through five years	15,319
After five years through ten years	1,563
After ten years	14,428
Fair/Book value, excluding investment securities not due at a single maturity date	187,244
Investment securities not due at a single maturity date, mortgage-backed securities	271,067
Fair/Book value	458,311	481,596	456,323
Book Value [Abstract]
Within one year	2,085
After one year through five years	6,730
After five years through ten years	20,230
After ten years	22,414
Book value, excluding investment securities not due at a single maturity date	51,459
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	51,459	54,460	55,542
Fair Value [Abstract]
Within one year	2,086
After one year through five years	6,733
After five years through ten years	20,210
After ten years	22,353
Fair value, excluding investment securities not due at a single maturity date	51,382
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 51,382	$ 55,236	$ 56,291