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Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Details)
$ in Thousands
Mar. 31, 2016
USD ($)
Investment
Dec. 31, 2015
USD ($)
Investment
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 8,548,238 $ 8,790,963
Unrealized Gains 89,141 16,694
Unrealized Losses (6,427) (49,035)
Fair Value 8,630,952 8,758,622
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 449,217 6,302,783
Unrealized Losses, Less than 12 months (2,152) (44,391)
Fair Value of Investments, 12 months or longer 456,725 238,350
Unrealized Losses, 12 months or longer (4,275) (4,644)
Fair Value of Investments 905,942 6,541,133
Unrealized Losses $ (6,427) $ (49,035)
Number of held-to-maturity investments with unrealized loss | Investment 161 384.000
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 72 58.000
U.S. agency debentures    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 589,076 $ 545,473
Unrealized Gains 15,170 8,876
Unrealized Losses 0 0
Fair Value 604,246 554,349
Agency-issued mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 2,308,454 2,366,627
Unrealized Gains 26,627 546
Unrealized Losses (622) (11,698)
Fair Value 2,334,459 2,355,475
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 31,969 2,121,258
Unrealized Losses, Less than 12 months (147) (10,860)
Fair Value of Investments, 12 months or longer 42,989 22,507
Unrealized Losses, 12 months or longer (475) (838)
Fair Value of Investments 74,958 2,143,765
Unrealized Losses (622) (11,698)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 4,047,342 4,225,781
Unrealized Gains 32,071 3,054
Unrealized Losses (4,483) (32,999)
Fair Value 4,074,930 4,195,836
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 181,655 3,153,483
Unrealized Losses, Less than 12 months (1,466) (30,230)
Fair Value of Investments, 12 months or longer 358,847 150,058
Unrealized Losses, 12 months or longer (3,017) (2,769)
Fair Value of Investments 540,502 3,303,541
Unrealized Losses (4,483) (32,999)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 359,244 370,779
Unrealized Gains 387 758
Unrealized Losses (352) (33)
Fair Value 359,279 371,504
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 177,805 170,350
Unrealized Losses, Less than 12 months (352) 33
Fair Value of Investments, 12 months or longer 0 0
Unrealized Losses, 12 months or longer 0 0
Fair Value of Investments 177,805 170,350
Unrealized Losses (352) 33
Agency-issued commercial mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,181,904 1,214,716
Unrealized Gains 14,773 3,405
Unrealized Losses (203) (3,475)
Fair Value 1,196,474 1,214,646
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 31,913 823,414
Unrealized Losses, Less than 12 months (19) (2,994)
Fair Value of Investments, 12 months or longer 26,907 40,276
Unrealized Losses, 12 months or longer (184) (481)
Fair Value of Investments 58,820 863,690
Unrealized Losses (203) (3,475)
Municipal bonds and notes    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 62,218 67,587
Unrealized Gains 113 55
Unrealized Losses (767) (830)
Fair Value 61,564 66,812
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 25,875 34,278
Unrealized Losses, Less than 12 months (168) (274)
Fair Value of Investments, 12 months or longer 27,982 25,509
Unrealized Losses, 12 months or longer (599) (556)
Fair Value of Investments 53,857 59,787
Unrealized Losses $ (767) $ (830)