Investment Securities, Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Amortized Cost [Abstract]
Within one year	$ 51,503
After one year through five years	5,100
After five years through ten years	563
After ten years	6,161
Amortized cost, excluding investment securities not due at a single maturity date	63,327
Investment securities not due at a single maturity date, mortgage-backed securities	270,667
Amortized cost	333,994	$ 789,175	$ 550,053
Fair Value [Abstract]
Within one year	51,569
After one year through five years	5,262
After five years through ten years	566
After ten years	6,185
Fair value, excluding investment securities not due at a single maturity date	63,582
Investment securities not due at a single maturity date, mortgage-backed securities	269,569
Fair value	333,151	807,732	568,536
Amortized Cost [Abstract]
Within one year	308
After one year through five years	6,969
After five years through ten years	19,543
After ten years	36,116
Book value, excluding investment securities not due at a single maturity date	62,936
Investment securities not due at a single maturity date, mortgage-backed securities	487,682
Amortized cost	550,618	68,933	69,913
Fair Value [Abstract]
Within one year	308
After one year through five years	7,012
After five years through ten years	20,254
After ten years	36,130
Fair value, excluding investment securities not due at a single maturity date	63,704
Investment securities not due at a single maturity date, mortgage-backed securities	476,604
Fair value	$ 540,308	$ 70,049	$ 71,055