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Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Details)
$ in Thousands
Sep. 30, 2016
USD ($)
Investment
Dec. 31, 2015
USD ($)
Investment
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 7,791,949 $ 8,790,963
Unrealized Gains 97,581 16,694
Unrealized Losses (4,197) (49,035)
Fair Value 7,885,333 8,758,622
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 420,064 6,302,783
Unrealized Losses, Less than 12 months (1,057) (44,391)
Fair Value of Investments, 12 months or longer 323,211 238,350
Unrealized Losses, 12 months or longer (3,140) (4,644)
Fair Value of Investments 743,275 6,541,133
Unrealized Losses $ (4,197) $ (49,035)
Number of held-to-maturity investments with unrealized loss | Investment 140 384
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 78 58
U.S. agency debentures    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 622,692 $ 545,473
Unrealized Gains 17,642 8,876
Unrealized Losses 0 0
Fair Value 640,334 554,349
Agency-issued mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 2,092,082 2,366,627
Unrealized Gains 37,993 546
Unrealized Losses (309) (11,698)
Fair Value 2,129,766 2,355,475
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 8,249 2,121,258
Unrealized Losses, Less than 12 months (53) (10,860)
Fair Value of Investments, 12 months or longer 23,238 22,507
Unrealized Losses, 12 months or longer (256) (838)
Fair Value of Investments 31,487 2,143,765
Unrealized Losses (309) (11,698)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 3,593,453 4,225,781
Unrealized Gains 28,800 3,054
Unrealized Losses (1,962) (32,999)
Fair Value 3,620,291 4,195,836
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 77,313 3,153,483
Unrealized Losses, Less than 12 months (136) (30,230)
Fair Value of Investments, 12 months or longer 247,706 150,058
Unrealized Losses, 12 months or longer (1,826) (2,769)
Fair Value of Investments 325,019 3,303,541
Unrealized Losses (1,962) (32,999)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 327,673 370,779
Unrealized Gains 188 758
Unrealized Losses (580) (33)
Fair Value 327,281 371,504
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 207,339 170,350
Unrealized Losses, Less than 12 months (580) (33)
Fair Value of Investments, 12 months or longer 0 0
Unrealized Losses, 12 months or longer 0 0
Fair Value of Investments 207,339 170,350
Unrealized Losses (580) (33)
Agency-issued commercial mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,101,234 1,214,716
Unrealized Gains 12,837 3,405
Unrealized Losses (311) (3,475)
Fair Value 1,113,760 1,214,646
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 112,586 823,414
Unrealized Losses, Less than 12 months (157) (2,994)
Fair Value of Investments, 12 months or longer 19,272 40,276
Unrealized Losses, 12 months or longer (154) (481)
Fair Value of Investments 131,858 863,690
Unrealized Losses (311) (3,475)
Municipal bonds and notes    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 54,815 67,587
Unrealized Gains 121 55
Unrealized Losses (1,035) (830)
Fair Value 53,901 66,812
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 14,577 34,278
Unrealized Losses, Less than 12 months (131) (274)
Fair Value of Investments, 12 months or longer 32,995 25,509
Unrealized Losses, 12 months or longer (904) (556)
Fair Value of Investments 47,572 59,787
Unrealized Losses $ (1,035) $ (830)