Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Amortized Cost [Abstract]
Within one year	$ 123,199
After one year through five years	14,798
After five years through ten years	17,877
After ten years	22,137
Amortized cost, excluding investment securities not due at a single maturity date	178,011
Investment securities not due at a single maturity date, mortgage-backed securities	324,251
Amortized cost	502,262
Fair/Book Value [Abstract]
Within one year	122,897
After one year through five years	14,762
After five years through ten years	17,482
After ten years	21,910
Fair/Book value, excluding investment securities not due at a single maturity date	177,051
Investment securities not due at a single maturity date, mortgage-backed securities	318,763
Fair/Book value	495,814	$ 481,596	$ 452,842
Book Value [Abstract]
Within one year	2,907
After one year through five years	6,711
After five years through ten years	17,707
After ten years	26,202
Book value, excluding investment securities not due at a single maturity date	53,527
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	53,527	54,460	57,281
Fair Value [Abstract]
Within one year	2,915
After one year through five years	6,714
After five years through ten years	17,795
After ten years	26,321
Fair value, excluding investment securities not due at a single maturity date	53,745
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 53,745	$ 55,236	$ 57,673