Securities - Amortized Cost and Fair Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost
Due within one year	$ 9,258
Due after one year through five years	38,526
Due after five years through ten years	71,434
Due after ten years	194,270
Amortized Cost	1,253,273	$ 1,276,871
Fair Value
Due within one year	9,316
Due after one year through five years	40,210
Due after five years through ten years	75,283
Due after ten years	196,052
Fair Value	1,293,388	1,290,613
Other debt securities
Amortized Cost
Debt securities, without fixed maturity, amortized cost	36,688
Amortized Cost	62,495	55,364
Fair Value
Debt securities, without fixed maturity, fair value	39,197
Fair Value	65,299	56,365
Government agency mortgage backed securities \| Residential mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	649,383
Amortized Cost	649,383	708,970
Fair Value
Debt securities, without fixed maturity, fair value	672,920
Fair Value	672,920	716,105
Government agency mortgage backed securities \| Commercial mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	29,198
Amortized Cost	29,198	30,372
Fair Value
Debt securities, without fixed maturity, fair value	30,513
Fair Value	30,513	31,007
Government agency collateralized mortgage obligations \| Residential mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	133,993
Amortized Cost	133,993	172,178
Fair Value
Debt securities, without fixed maturity, fair value	136,145
Fair Value	136,145	173,238
Government agency collateralized mortgage obligations \| Commercial mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	90,523
Amortized Cost	90,523	76,456
Fair Value
Debt securities, without fixed maturity, fair value	93,752
Fair Value	$ 93,752	$ 77,751

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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