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Debt Securities Available-for-Sale - Summary of Expected Maturity Distribution of Debt Securities Available-for-Sale, Other than Mortgage-Backed Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized cost
Amortized cost	$ 1,083,047	$ 1,246,673
Estimated fair value
Estimated fair value	1,097,423	$ 1,264,805
Other Than Mortgage Backed Securities
Amortized cost
Due in one year or less	35,282
Due after one year through five years	73,463
Due after five years through ten years	17,768
Amortized cost	126,513
Estimated fair value
Due in one year or less	35,757
Due after one year through five years	73,804
Due after five years through ten years	17,717
Estimated fair value	$ 127,278

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