Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Amortized Cost [Abstract]
Within one year	$ 5,642
After one year through five years	32,404
After five years through ten years	23,307
After ten years	7,069
Amortized cost, excluding investment securities not due at a single maturity date	68,422
Investment securities not due at a single maturity date, mortgage-backed securities	494,399
Amortized cost	562,821	$ 789,175	$ 512,776
Fair Value [Abstract]
Within one year	5,650
After one year through five years	32,682
After five years through ten years	23,143
After ten years	6,987
Fair value, excluding investment securities not due at a single maturity date	68,462
Investment securities not due at a single maturity date, mortgage-backed securities	492,511
Fair value	560,973	807,732	532,246
Amortized Cost [Abstract]
Within one year	8,152
After one year through five years	5,542
After five years through ten years	22,277
After ten years	34,483
Book value, excluding investment securities not due at a single maturity date	70,454
Investment securities not due at a single maturity date, mortgage-backed securities	314,600
Amortized cost	385,054	68,933	58,701
Fair Value [Abstract]
Within one year	8,152
After one year through five years	5,582
After five years through ten years	23,052
After ten years	34,573
Fair value, excluding investment securities not due at a single maturity date	71,359
Investment securities not due at a single maturity date, mortgage-backed securities	303,469
Fair value	$ 374,828	$ 70,049	$ 59,503