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Investment Securities - Summary of Unrealized Losses on Available-for-Sale Securities (Details)
$ in Thousands
Mar. 31, 2019
USD ($)
Investment
Dec. 31, 2018
USD ($)
Investment
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments $ 114,742 $ 957,319
Less than 12 months - Unrealized Losses (39) (5,412)
12 months or longer - Fair Value of Investments 5,586,169 6,039,639
12 months or longer - Unrealized Losses (41,825) (78,938)
Fair Value of Investments 5,700,911 6,996,958
Unrealized Losses $ (41,864) $ (84,350)
Number of investments in unrealized loss position | Investment 159 200
Number of investments with unrealized losses greater than 12 months | Investment 151 162
U.S. treasury securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments $ 101,342 $ 494,287
Less than 12 months - Unrealized Losses (28) (3,785)
12 months or longer - Fair Value of Investments 2,878,665 3,568,119
12 months or longer - Unrealized Losses (19,764) (31,777)
Fair Value of Investments 2,980,007 4,062,406
Unrealized Losses (19,792) (35,562)
U.S. agency debentures    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 0 443,790
Less than 12 months - Unrealized Losses 0 (1,602)
12 months or longer - Fair Value of Investments 865,746 591,216
12 months or longer - Unrealized Losses (2,960) (4,768)
Fair Value of Investments 865,746 1,035,006
Unrealized Losses (2,960) (6,370)
Foreign government debt securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 5,685 5,812
Less than 12 months - Unrealized Losses (4) (3)
12 months or longer - Fair Value of Investments 0 0
12 months or longer - Unrealized Losses 0 0
Fair Value of Investments 5,685 5,812
Unrealized Losses (4) (3)
Residential mortgage-backed securities | Fixed rate    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 5,085 13,430
Less than 12 months - Unrealized Losses 0 (22)
12 months or longer - Fair Value of Investments 1,834,798 1,866,788
12 months or longer - Unrealized Losses (19,096) (42,378)
Fair Value of Investments 1,839,883 1,880,218
Unrealized Losses (19,096) (42,400)
Residential mortgage-backed securities | Variable rate    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 2,630 0
Less than 12 months - Unrealized Losses (7) 0
12 months or longer - Fair Value of Investments 6,960 13,516
12 months or longer - Unrealized Losses (5) (15)
Fair Value of Investments 9,590 13,516
Unrealized Losses $ (12) $ (15)