Securities - Amortized Cost and Fair Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Due within one year	$ 36,914
Due after one year through five years	23,273
Due after five years through ten years	57,089
Due after ten years	228,196
Amortized Cost	2,138,772	$ 1,301,973
Fair Value
Due within one year	39,268
Due after one year through five years	24,061
Due after five years through ten years	59,654
Due after ten years	234,605
Fair Value	2,163,820	1,343,457
Other debt securities
Amortized Cost
Debt securities, without fixed maturity, amortized cost	31,747
Amortized Cost	59,523	62,771
Fair Value
Debt securities, without fixed maturity, fair value	33,685
Fair Value	62,716	65,607
Government agency mortgage backed securities \| Residential mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	742,392
Amortized Cost	742,392	581,105
Fair Value
Debt securities, without fixed maturity, fair value	754,490
Fair Value	754,490	602,646
Government agency mortgage backed securities \| Commercial mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	21,461
Amortized Cost	21,461	29,053
Fair Value
Debt securities, without fixed maturity, fair value	22,256
Fair Value	22,256	30,287
Government agency collateralized mortgage obligations \| Residential mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	800,299
Amortized Cost	800,299	218,373
Fair Value
Debt securities, without fixed maturity, fair value	798,242
Fair Value	798,242	220,268
Government agency collateralized mortgage obligations \| Commercial mortgage backed securities:
Amortized Cost
Debt securities, without fixed maturity, amortized cost	197,401
Amortized Cost	197,401	99,377
Fair Value
Debt securities, without fixed maturity, fair value	197,559
Fair Value	$ 197,559	$ 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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