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Investment Securities - Summary of Unrealized Losses on Available-for-Sale Securities (Details)
$ in Thousands
Sep. 30, 2020
USD ($)
Investment
Dec. 31, 2019
USD ($)
Investment
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments $ 2,493,315 $ 2,102,365
Less than 12 months - Unrealized Losses (8,578) (20,007)
12 months or longer - Fair Value of Investments 0 449,850
12 months or longer - Unrealized Losses 0 (135)
Fair Value of Investments 2,493,315 2,552,215
Unrealized Losses $ (8,578) $ (20,142)
Number of investments in unrealized loss position | Investment 57 58
Number of investments with unrealized losses greater than 12 months | Investment 0 12
U.S. treasury securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments   $ 971,572
Less than 12 months - Unrealized Losses   (3,996)
12 months or longer - Fair Value of Investments   449,850
12 months or longer - Unrealized Losses   (135)
Fair Value of Investments   1,421,422
Unrealized Losses   (4,131)
U.S. agency debentures    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments   99,547
Less than 12 months - Unrealized Losses   (453)
12 months or longer - Fair Value of Investments   0
12 months or longer - Unrealized Losses   0
Fair Value of Investments   99,547
Unrealized Losses   (453)
Foreign government debt securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments $ 23,449  
Less than 12 months - Unrealized Losses (3)  
12 months or longer - Fair Value of Investments 0  
12 months or longer - Unrealized Losses 0  
Fair Value of Investments 23,449  
Unrealized Losses (3)  
Residential mortgage-backed securities | Agency-issued mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 179,050 4,014
Less than 12 months - Unrealized Losses (102) (19)
12 months or longer - Fair Value of Investments 0 0
12 months or longer - Unrealized Losses 0 0
Fair Value of Investments 179,050 4,014
Unrealized Losses (102) (19)
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 1,772,593  
Less than 12 months - Unrealized Losses (7,488)  
12 months or longer - Fair Value of Investments 0  
12 months or longer - Unrealized Losses 0  
Fair Value of Investments 1,772,593  
Unrealized Losses (7,488)  
Agency-issued commercial mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 518,223 1,027,232
Less than 12 months - Unrealized Losses (985) (15,539)
12 months or longer - Fair Value of Investments 0 0
12 months or longer - Unrealized Losses 0 0
Fair Value of Investments 518,223 1,027,232
Unrealized Losses $ (985) $ (15,539)