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Investment Securities - Summary of Unrealized Losses on Available-for-Sale Securities (Detail)
$ in Thousands
Sep. 30, 2018
USD ($)
Investment
Dec. 31, 2017
USD ($)
Investment
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments $ 5,580,693 $ 8,913,199
Less than 12 months - Unrealized Losses (81,473) (51,432)
12 months or longer - Fair Value of Investments 3,100,433 713,348
12 months or longer - Unrealized Losses (68,636) (5,752)
Fair Value of Investments 8,681,126 9,626,547
Unrealized Losses $ (150,109) $ (57,184)
Number of investments in unrealized loss position | Investment 265 268
Number of investments with unrealized losses greater than 12 months | Investment 109 46
U.S. treasury securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments $ 3,915,282 $ 5,968,914
Less than 12 months - Unrealized Losses (46,503) (23,397)
12 months or longer - Fair Value of Investments 1,438,839 323,966
12 months or longer - Unrealized Losses (12,568) (2,282)
Fair Value of Investments 5,354,121 6,292,880
Unrealized Losses (59,071) (25,679)
U.S. agency debentures    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 838,609 736,541
Less than 12 months - Unrealized Losses (4,615) (2,289)
12 months or longer - Fair Value of Investments 508,062 336,196
12 months or longer - Unrealized Losses (5,782) (3,347)
Fair Value of Investments 1,346,671 1,072,737
Unrealized Losses (10,397) (5,636)
Residential mortgage-backed securities | Fixed rate | Agency-issued collateralized mortgage obligations    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 812,726 2,193,277
Less than 12 months - Unrealized Losses (30,350) (25,534)
12 months or longer - Fair Value of Investments 1,103,803 0
12 months or longer - Unrealized Losses (50,160) 0
Fair Value of Investments 1,916,529 2,193,277
Unrealized Losses (80,510) (25,534)
Residential mortgage-backed securities | Variable rate | Agency-issued collateralized mortgage obligations    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments 14,076 13,843
Less than 12 months - Unrealized Losses (5) (3)
12 months or longer - Fair Value of Investments 49,729 53,186
12 months or longer - Unrealized Losses (126) (123)
Fair Value of Investments 63,805 67,029
Unrealized Losses $ (131) (126)
Equity securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months - Fair Value of Investments   624
Less than 12 months - Unrealized Losses   (209)
12 months or longer - Fair Value of Investments   0
12 months or longer - Unrealized Losses   0
Fair Value of Investments   624
Unrealized Losses   $ (209)