Securities - Amortized Cost and Fair Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Due within one year	$ 9,229
Due after one year through five years	42,312
Due after five years through ten years	58,334
Due after ten years	290,472
Amortized Cost	2,532,767	$ 1,301,973
Fair Value
Due within one year	9,334
Due after one year through five years	44,024
Due after five years through ten years	61,568
Due after ten years	293,271
Fair Value	2,544,643	1,343,457
Other debt securities
Amortized Cost
Debt securities, without fixed maturity, amortized cost	26,299
Amortized Cost	51,321	62,771
Fair Value
Debt securities, without fixed maturity, fair value	27,775
Fair Value	53,967	65,607
Government agency mortgage backed securities \| Residential mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	935,233
Amortized Cost	935,233	581,105
Fair Value
Debt securities, without fixed maturity, fair value	943,238
Fair Value	943,238	602,646
Government agency mortgage backed securities \| Commercial mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	16,650
Amortized Cost	16,650	29,053
Fair Value
Debt securities, without fixed maturity, fair value	17,164
Fair Value	17,164	30,287
Government agency collateralized mortgage obligations \| Residential mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	962,684
Amortized Cost	962,684	218,373
Fair Value
Debt securities, without fixed maturity, fair value	957,737
Fair Value	957,737	220,268
Government agency collateralized mortgage obligations \| Commercial mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	191,554
Amortized Cost	191,554	99,377
Fair Value
Debt securities, without fixed maturity, fair value	190,532
Fair Value	$ 190,532	$ 102,348

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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