Investment Securities - Contractual Maturities of Debt Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year	$ 10,183
Due after one year through five years	31,000
Due after five years through ten years	84,781
Due after ten years	85,283
Residential mortgage-backed securities	973,543
Amortized Cost	1,184,790	$ 1,293,506
Fair Value
Due in one year	10,294
Due after one year through five years	31,750
Due after five years through ten years	87,587
Due after ten years	85,818
Residential mortgage-backed securities	992,828
Total	$ 1,208,277	$ 1,339,473

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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