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Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Details)
$ in Thousands
Jun. 30, 2016
USD ($)
Investment
Dec. 31, 2015
USD ($)
Investment
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 8,200,443 $ 8,790,963
Unrealized Gains 124,196 16,694
Unrealized Losses (2,591) (49,035)
Fair Value 8,322,048 8,758,622
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 199,491 6,302,783
Unrealized Losses, Less than 12 months (136) (44,391)
Fair Value of Investments, 12 months or longer 356,061 238,350
Unrealized Losses, 12 months or longer (2,455) (4,644)
Fair Value of Investments 555,552 6,541,133
Unrealized Losses $ (2,591) $ (49,035)
Number of held-to-maturity investments with unrealized loss | Investment 132 384
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 81 58
U.S. agency debentures    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 588,813 $ 545,473
Unrealized Gains 19,436 8,876
Unrealized Losses 0 0
Fair Value 608,249 554,349
Agency-issued mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 2,228,633 2,366,627
Unrealized Gains 40,751 546
Unrealized Losses (270) (11,698)
Fair Value 2,269,114 2,355,475
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 766 2,121,258
Unrealized Losses, Less than 12 months (20) (10,860)
Fair Value of Investments, 12 months or longer 24,980 22,507
Unrealized Losses, 12 months or longer (250) (838)
Fair Value of Investments 25,746 2,143,765
Unrealized Losses (270) (11,698)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 3,829,568 4,225,781
Unrealized Gains 44,721 3,054
Unrealized Losses (1,267) (32,999)
Fair Value 3,873,022 4,195,836
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 0 3,153,483
Unrealized Losses, Less than 12 months 0 (30,230)
Fair Value of Investments, 12 months or longer 270,867 150,058
Unrealized Losses, 12 months or longer (1,267) (2,769)
Fair Value of Investments 270,867 3,303,541
Unrealized Losses (1,267) (32,999)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 343,040 370,779
Unrealized Gains 200 758
Unrealized Losses (41) (33)
Fair Value 343,199 371,504
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 186,137 170,350
Unrealized Losses, Less than 12 months (41) (33)
Fair Value of Investments, 12 months or longer 0 0
Unrealized Losses, 12 months or longer 0 0
Fair Value of Investments 186,137 170,350
Unrealized Losses (41) (33)
Agency-issued commercial mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,152,105 1,214,716
Unrealized Gains 18,927 3,405
Unrealized Losses (164) (3,475)
Fair Value 1,170,868 1,214,646
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 0 823,414
Unrealized Losses, Less than 12 months 0 (2,994)
Fair Value of Investments, 12 months or longer 26,424 40,276
Unrealized Losses, 12 months or longer (164) (481)
Fair Value of Investments 26,424 863,690
Unrealized Losses (164) (3,475)
Municipal bonds and notes    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 58,284 67,587
Unrealized Gains 161 55
Unrealized Losses (849) (830)
Fair Value 57,596 66,812
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 12,588 34,278
Unrealized Losses, Less than 12 months (75) (274)
Fair Value of Investments, 12 months or longer 33,790 25,509
Unrealized Losses, 12 months or longer (774) (556)
Fair Value of Investments 46,378 59,787
Unrealized Losses $ (849) $ (830)