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Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Detail)
$ in Thousands
Jun. 30, 2018
USD ($)
Investment
Dec. 31, 2017
USD ($)
Investment
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months $ 11,369,488 $ 6,710,931
Unrealized Losses, Less than 12 months (295,241) (62,993)
Fair Value of Investments, 12 months or longer 2,785,559 2,966,895
Unrealized Losses, 12 months or longer (112,599) (65,105)
Fair Value of Investments 14,155,047 9,677,826
Unrealized Losses $ (407,840) $ (128,098)
Number of held-to-maturity investments with unrealized loss | Investment 1,236 753
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 226 237
U.S. agency debentures    
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months $ 645,990 $ 104,688
Unrealized Losses, Less than 12 months (10,883) (1,601)
Fair Value of Investments, 12 months or longer 54,001 0
Unrealized Losses, 12 months or longer (2,935) 0
Fair Value of Investments 699,991 104,688
Unrealized Losses (13,818) (1,601)
Agency-issued mortgage-backed securities    
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 7,436,980 4,270,377
Unrealized Losses, Less than 12 months (192,418) (34,092)
Fair Value of Investments, 12 months or longer 364,687 408,913
Unrealized Losses, 12 months or longer (17,730) (9,436)
Fair Value of Investments 7,801,667 4,679,290
Unrealized Losses (210,148) (43,528)
Residential mortgage-backed securities | Fixed rate | Agency-issued collateralized mortgage obligations    
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 776,389 1,011,709
Unrealized Losses, Less than 12 months (24,833) (13,631)
Fair Value of Investments, 12 months or longer 1,626,527 1,741,614
Unrealized Losses, 12 months or longer (67,086) (40,741)
Fair Value of Investments 2,402,916 2,753,323
Unrealized Losses (91,919) (54,372)
Residential mortgage-backed securities | Variable rate | Agency-issued collateralized mortgage obligations    
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 3,687 0
Unrealized Losses, Less than 12 months (1) 0
Fair Value of Investments, 12 months or longer 8,889 9,812
Unrealized Losses, 12 months or longer (30) (34)
Fair Value of Investments 12,576 9,812
Unrealized Losses (31) (34)
Agency-issued commercial mortgage-backed securities    
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 1,460,340 979,361
Unrealized Losses, Less than 12 months (38,747) (11,566)
Fair Value of Investments, 12 months or longer 715,596 773,712
Unrealized Losses, 12 months or longer (24,235) (13,997)
Fair Value of Investments 2,175,936 1,753,073
Unrealized Losses (62,982) (25,563)
Municipal bonds and notes    
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 1,046,102 344,796
Unrealized Losses, Less than 12 months (28,359) (2,103)
Fair Value of Investments, 12 months or longer 15,859 32,844
Unrealized Losses, 12 months or longer (583) (897)
Fair Value of Investments 1,061,961 377,640
Unrealized Losses $ (28,942) $ (3,000)