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Investment Securities - Summary of Unrealized Losses on Available-for-Sale Securities (Details)
$ in Thousands
Jun. 30, 2019
USD ($)
Investment
Dec. 31, 2018
USD ($)
Investment
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments $ 8,220 $ 957,319
Less than 12 months - Unrealized Losses (22) (5,412)
12 months or longer - Fair Value of Investments 2,344,091 6,039,639
12 months or longer - Unrealized Losses (5,424) (78,938)
Fair Value of Investments 2,352,311 6,996,958
Unrealized Losses $ (5,446) $ (84,350)
Number of investments in unrealized loss position | Investment 57 200
Number of investments with unrealized losses greater than 12 months | Investment 54 162
U.S. treasury securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments $ 0 $ 494,287
Less than 12 months - Unrealized Losses 0 (3,785)
12 months or longer - Fair Value of Investments 2,318,760 3,568,119
12 months or longer - Unrealized Losses (5,369) (31,777)
Fair Value of Investments 2,318,760 4,062,406
Unrealized Losses (5,369) (35,562)
U.S. agency debentures    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments   443,790
Less than 12 months - Unrealized Losses   (1,602)
12 months or longer - Fair Value of Investments   591,216
12 months or longer - Unrealized Losses   (4,768)
Fair Value of Investments   1,035,006
Unrealized Losses   (6,370)
Foreign government debt securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments   5,812
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   5,812
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 8,220  
Less than 12 months - Unrealized Losses (22)  
12 months or longer - Fair Value of Investments 0  
12 months or longer - Unrealized Losses 0  
Fair Value of Investments 8,220  
Unrealized Losses (22)  
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 0 13,430
Less than 12 months - Unrealized Losses 0 (22)
12 months or longer - Fair Value of Investments 25,331 1,866,788
12 months or longer - Unrealized Losses (55) (42,378)
Fair Value of Investments 25,331 1,880,218
Unrealized Losses $ (55) (42,400)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—variable rate    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments   0
Less than 12 months - Unrealized Losses   0
12 months or longer - Fair Value of Investments   13,516
12 months or longer - Unrealized Losses   (15)
Fair Value of Investments   13,516
Unrealized Losses   $ (15)