Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost [Abstract]
Within one year	$ 5,479
After one year through five years	25,427
After five years through ten years	30,281
After ten years	7,426
Amortized cost, excluding securities without single maturity date	68,613
Investment securities not due at a single maturity date, mortgage-backed securities	720,562
Amortized cost	789,175	$ 502,693
Fair Value [Abstract]
Within one year	5,512
After one year through five years	26,096
After five years through ten years	30,912
After ten years	7,339
Fair value, excluding investment securities not due at a single maturity date	69,859
Investment securities not due at a single maturity date, mortgage-backed securities	737,873
Fair value	807,732	507,386
Amortized Cost [Abstract]
Within one year	8,309
After one year through five years	5,137
After five years through ten years	23,493
After ten years	31,994
Book value, excluding securities without single maturity date	68,933
Investment securities not due at a single maturity date, mortgage-backed securities	0
Amortized cost	68,933	60,229
Fair Value [Abstract]
Within one year	8,309
After one year through five years	5,179
After five years through ten years	24,451
After ten years	32,110
Fair/Book value, excluding securities without single maturity date	70,049
Investment securities not due at a single maturity date, mortgage-backed securities	0
Fair value	$ 70,049	$ 61,097