Debt Securities Available for Sale - Expected Maturities of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost
Amortized Cost	$ 1,204,001	$ 1,086,143
Fair Value
Fair Value	1,245,300	1,098,336
Debt Securities excluding Mortgage-based securities and CMOs
Amortized Cost
One year or less	16,399
More than one year to five years	42,422
More than five years to ten years	49,590
More than ten years	470
Amortized Cost	108,881
Fair Value
One year or less	16,416
More than one year to five years	44,295
More than five years to ten years	50,000
More than ten years	483
Fair Value	111,194
Mortgage-backed securities and collateralized mortgage obligations
Amortized Cost
Amortized Cost	1,095,120	968,165
Fair Value
Fair Value	$ 1,134,106	$ 979,881

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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