Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Amortized Cost [Abstract]
Within one year	$ 118,646
After one year through five years	15,118
After five years through ten years	1,126
After ten years	10,318
Amortized cost, excluding investment securities not due at a single maturity date	145,208
Investment securities not due at a single maturity date, mortgage-backed securities	343,295
Amortized cost	488,503	$ 499,639	$ 467,628
Fair Value [Abstract]
Within one year	118,641
After one year through five years	15,387
After five years through ten years	1,126
After ten years	10,219
Fair value, excluding investment securities not due at a single maturity date	145,373
Investment securities not due at a single maturity date, mortgage-backed securities	347,557
Fair value	492,930	495,396	458,311
Book Value [Abstract]
Within one year	2,151
After one year through five years	4,696
After five years through ten years	25,426
After ten years	28,081
Book value, excluding investment securities not due at a single maturity date	60,354
Investment securities not due at a single maturity date, mortgage-backed securities	0
Book value	60,354	53,566	51,459
Fair Value [Abstract]
Within one year	2,152
After one year through five years	4,702
After five years through ten years	26,174
After ten years	28,265
Fair value, excluding investment securities not due at a single maturity date	61,293
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 61,293	$ 53,738	$ 51,382