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Investment Securities - Summary of Unrealized Losses on Available-for-Sale Securities (Details)
$ in Thousands
Mar. 31, 2021
USD ($)
Investment
Dec. 31, 2020
USD ($)
Investment
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments $ 16,061,990 $ 4,285,435
Less than 12 months - Unrealized Losses (498,349) (14,988)
12 months or longer - Fair Value of Investments 0 0
12 months or longer - Unrealized Losses 0 0
Fair Value of Investments 16,061,990 4,285,435
Unrealized Losses $ (498,349) $ (14,988)
Number of investments in unrealized loss position | Investment 400 93
Number of investments with unrealized losses greater than 12 months | Investment 0 0
U.S. treasury securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments $ 301,607 $ 59,929
Less than 12 months - Unrealized Losses (1,223) (107)
12 months or longer - Fair Value of Investments 0 0
12 months or longer - Unrealized Losses 0 0
Fair Value of Investments 301,607 59,929
Unrealized Losses (1,223) (107)
U.S. agency debentures    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 128,228 133,143
Less than 12 months - Unrealized Losses (5,435) (585)
12 months or longer - Fair Value of Investments 0 0
12 months or longer - Unrealized Losses 0 0
Fair Value of Investments 128,228 133,143
Unrealized Losses (5,435) (585)
Foreign government debt securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 23,450  
Less than 12 months - Unrealized Losses (4)  
12 months or longer - Fair Value of Investments 0  
12 months or longer - Unrealized Losses 0  
Fair Value of Investments 23,450  
Unrealized Losses (4)  
Residential mortgage-backed securities | Agency-issued mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 9,608,598 903,767
Less than 12 months - Unrealized Losses (233,388) (651)
12 months or longer - Fair Value of Investments 0 0
12 months or longer - Unrealized Losses 0 0
Fair Value of Investments 9,608,598 903,767
Unrealized Losses (233,388) (651)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 5,248,844 2,199,207
Less than 12 months - Unrealized Losses (234,507) (10,278)
12 months or longer - Fair Value of Investments 0 0
12 months or longer - Unrealized Losses 0 0
Fair Value of Investments 5,248,844 2,199,207
Unrealized Losses (234,507) (10,278)
Agency-issued commercial mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 751,263 989,389
Less than 12 months - Unrealized Losses (23,792) (3,367)
12 months or longer - Fair Value of Investments 0 0
12 months or longer - Unrealized Losses 0 0
Fair Value of Investments 751,263 989,389
Unrealized Losses $ (23,792) $ (3,367)