Investment Securities - Amortized Cost and Fair Value of Investment Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 453
Due after one year through five years	13,395
Due after five years through ten years	109,852
Due after ten years	176,028
Total amortized cost	299,728
Mortgage-backed securities	525,507
Amortized Cost	825,235	$ 372,231
Fair Value
Due in one year or less	453
Due after one year through five years	13,457
Due after five years through ten years	108,879
Due after ten years	173,127
Total fair value	295,916
Mortgage-backed securities	524,709
Fair Value	$ 820,625	$ 380,814