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Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Details)
$ in Thousands
Mar. 31, 2018
USD ($)
Investment
Dec. 31, 2017
USD ($)
Investment
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 14,548,856 $ 12,663,455
Unrealized Gains 8,582 12,923
Unrealized Losses (327,999) (128,098)
Fair Value 14,229,439 12,548,280
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 9,383,909 6,710,931
Unrealized Losses, Less than 12 months (232,366) (62,993)
Fair Value of Investments, 12 months or longer 2,795,108 2,966,895
Unrealized Losses, 12 months or longer (95,633) (65,105)
Fair Value of Investments 12,179,017 9,677,826
Unrealized Losses $ (327,999) $ (128,098)
Number of held-to-maturity investments with unrealized loss | Investment 1,115 753
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 223 237
U.S. agency debentures    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 714,008 $ 659,979
Unrealized Gains 466 3,167
Unrealized Losses (6,900) (1,601)
Fair Value 707,574 661,545
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 624,806 104,688
Unrealized Losses, Less than 12 months (6,900) (1,601)
Fair Value of Investments, 12 months or longer 0 0
Unrealized Losses, 12 months or longer 0 0
Fair Value of Investments 624,806 104,688
Unrealized Losses (6,900) (1,601)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 8,043,443 6,304,969
Unrealized Gains 6,683 4,854
Unrealized Losses (160,250) (43,528)
Fair Value 7,889,876 6,266,295
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 5,974,994 4,270,377
Unrealized Losses, Less than 12 months (144,458) (34,092)
Fair Value of Investments, 12 months or longer 380,892 408,913
Unrealized Losses, 12 months or longer (15,792) (9,436)
Fair Value of Investments 6,355,886 4,679,290
Unrealized Losses (160,250) (43,528)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate    
Schedule of Held-to-maturity Securities [Line Items]    
Fair Value 2,583,897 2,775,630
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments 2,583,633 2,753,323
Unrealized Losses (81,612) (54,372)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate    
Schedule of Held-to-maturity Securities [Line Items]    
Fair Value 244,438 256,481
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, 12 months or longer   9,812
Unrealized Losses, 12 months or longer   (34)
Fair Value of Investments 9,317 9,812
Unrealized Losses (29) (34)
Agency-issued commercial mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,833,961 1,868,985
Unrealized Gains 0 694
Unrealized Losses (52,121) (25,563)
Fair Value 1,781,840 1,844,116
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 1,042,701 979,361
Unrealized Losses, Less than 12 months (30,719) (11,566)
Fair Value of Investments, 12 months or longer 739,139 773,712
Unrealized Losses, 12 months or longer (21,402) (13,997)
Fair Value of Investments 1,781,840 1,753,073
Unrealized Losses (52,121) (25,563)
US States and Political Subdivisions Debt Securities [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,048,210 743,761
Unrealized Gains 691 3,452
Unrealized Losses (27,087) (3,000)
Fair Value 1,021,814 744,213
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 803,271 344,796
Unrealized Losses, Less than 12 months (26,367) (2,103)
Fair Value of Investments, 12 months or longer 20,264 32,844
Unrealized Losses, 12 months or longer (720) (897)
Fair Value of Investments 823,535 377,640
Unrealized Losses (27,087) (3,000)
Fixed rate | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 2,665,485 2,829,979
Unrealized Gains 24 23
Unrealized Losses (81,612) (54,372)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 938,137 1,011,709
Unrealized Losses, Less than 12 months (23,922) (13,631)
Fair Value of Investments, 12 months or longer 1,645,496 1,741,614
Unrealized Losses, 12 months or longer (57,690) (40,741)
Variable rate | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 243,749 255,782
Unrealized Gains 718 733
Unrealized Losses (29) (34)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 0 0
Unrealized Losses, Less than 12 months 0 $ 0
Fair Value of Investments, 12 months or longer 9,317  
Unrealized Losses, 12 months or longer $ (29)