Securities - Amortized Cost and Fair Value of Debt Securities by Contractual Maturity - Securities Held-to-maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Held-to-maturity, Amortized Cost
Due after 3 months through one year	$ 537
Due after one year through five years	10,347
Due after five years through ten years	29,564
Due after ten years	16,549
Agency mortgage-backed securities	480,333
Total	537,330
Held-to-maturity, Estimated Fair Value
Due after 3 months through one year	549
Due after one year through five years	10,548
Due after five years through ten years	30,003
Due after ten years	16,886
Agency mortgage-backed securities	481,447
Total	$ 539,433	$ 304,927