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Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Detail)
$ in Thousands
Sep. 30, 2018
USD ($)
Investment
Dec. 31, 2017
USD ($)
Investment
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months $ 9,169,096 $ 6,710,931
Unrealized Losses, Less than 12 months (254,565) (62,993)
Fair Value of Investments, 12 months or longer 5,971,026 2,966,895
Unrealized Losses, 12 months or longer (273,838) (65,105)
Fair Value of Investments 15,140,122 9,677,826
Unrealized Losses $ (528,403) $ (128,098)
Number of held-to-maturity investments with unrealized loss | Investment 1,433 753
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 454 237
U.S. agency debentures    
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months $ 549,916 $ 104,688
Unrealized Losses, Less than 12 months (14,744) (1,601)
Fair Value of Investments, 12 months or longer 61,635 0
Unrealized Losses, 12 months or longer (3,738) 0
Fair Value of Investments 611,551 104,688
Unrealized Losses (18,482) (1,601)
Agency-issued mortgage-backed securities    
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 5,514,402 4,270,377
Unrealized Losses, Less than 12 months (155,580) (34,092)
Fair Value of Investments, 12 months or longer 2,541,848 408,913
Unrealized Losses, 12 months or longer (121,168) (9,436)
Fair Value of Investments 8,056,250 4,679,290
Unrealized Losses (276,748) (43,528)
Residential mortgage-backed securities | Fixed rate | Agency-issued collateralized mortgage obligations    
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 77,011 1,011,709
Unrealized Losses, Less than 12 months (2,285) (13,631)
Fair Value of Investments, 12 months or longer 2,153,494 1,741,614
Unrealized Losses, 12 months or longer (96,274) (40,741)
Fair Value of Investments 2,230,505 2,753,323
Unrealized Losses (98,559) (54,372)
Residential mortgage-backed securities | Variable rate | Agency-issued collateralized mortgage obligations    
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 3,504 0
Unrealized Losses, Less than 12 months (1) 0
Fair Value of Investments, 12 months or longer 8,445 9,812
Unrealized Losses, 12 months or longer (31) (34)
Fair Value of Investments 11,949 9,812
Unrealized Losses (32) (34)
Agency-issued commercial mortgage-backed securities    
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 1,595,146 979,361
Unrealized Losses, Less than 12 months (30,149) (11,566)
Fair Value of Investments, 12 months or longer 1,122,118 773,712
Unrealized Losses, 12 months or longer (48,539) (13,997)
Fair Value of Investments 2,717,264 1,753,073
Unrealized Losses (78,688) (25,563)
Municipal bonds and notes    
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 1,429,117 344,796
Unrealized Losses, Less than 12 months (51,806) (2,103)
Fair Value of Investments, 12 months or longer 83,486 32,844
Unrealized Losses, 12 months or longer (4,088) (897)
Fair Value of Investments 1,512,603 377,640
Unrealized Losses $ (55,894) $ (3,000)