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Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Details)
$ in Thousands
Sep. 30, 2017
USD ($)
Investment
Dec. 31, 2016
USD ($)
Investment
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 11,055,006 $ 8,426,998
Unrealized Gains 27,481 16,968
Unrealized Losses (59,072) (67,828)
Fair Value 11,023,415 8,376,138
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 7,155,457 5,943,190
Unrealized Losses, Less than 12 months (50,684) (60,189)
Fair Value of Investments, 12 months or longer 382,402 295,657
Unrealized Losses, 12 months or longer (8,388) (7,639)
Fair Value of Investments 7,537,859 6,238,847
Unrealized Losses $ (59,072) $ (67,828)
Number of held-to-maturity investments with unrealized loss | Investment 547 462
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 84 85
U.S. agency debentures    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost $ 660,193 $ 622,445
Unrealized Gains 6,775 7,840
Unrealized Losses (1,080) (1,198)
Fair Value 665,888 629,087
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 101,211 118,721
Unrealized Losses, Less than 12 months (1,080) (1,198)
Fair Value of Investments, 12 months or longer 0 0
Unrealized Losses, 12 months or longer 0 0
Fair Value of Investments 101,211 118,721
Unrealized Losses (1,080) (1,198)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 5,164,701 2,896,179
Unrealized Gains 15,764 6,919
Unrealized Losses (17,116) (24,526)
Fair Value 5,163,349 2,878,572
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 3,071,006 1,801,861
Unrealized Losses, Less than 12 months (16,251) (23,558)
Fair Value of Investments, 12 months or longer 25,004 21,917
Unrealized Losses, 12 months or longer (865) (968)
Fair Value of Investments 3,096,010 1,823,778
Unrealized Losses (17,116) (24,526)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 3,025,421 3,362,598
Unrealized Gains 707 788
Unrealized Losses (27,268) (31,274)
Fair Value 2,998,860 3,332,112
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 2,661,682 2,729,889
Unrealized Losses, Less than 12 months (21,842) (25,723)
Fair Value of Investments, 12 months or longer 235,320 228,220
Unrealized Losses, 12 months or longer (5,426) (5,551)
Fair Value of Investments 2,897,002 2,958,109
Unrealized Losses (27,268) (31,274)
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 269,495 312,665
Unrealized Gains 704 176
Unrealized Losses (37) (1,339)
Fair Value 270,162 311,502
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 0 251,012
Unrealized Losses, Less than 12 months 0 (1,339)
Fair Value of Investments, 12 months or longer 10,321 0
Unrealized Losses, 12 months or longer (37) 0
Fair Value of Investments 10,321 251,012
Unrealized Losses (37) (1,339)
Agency-issued commercial mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 1,554,220 1,151,363
Unrealized Gains 2,025 1,237
Unrealized Losses (11,697) (7,638)
Fair Value 1,544,548 1,144,962
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 1,210,351 999,440
Unrealized Losses, Less than 12 months (10,615) (7,494)
Fair Value of Investments, 12 months or longer 82,151 14,934
Unrealized Losses, 12 months or longer (1,082) (144)
Fair Value of Investments 1,292,502 1,014,374
Unrealized Losses (11,697) (7,638)
US States and Political Subdivisions Debt Securities [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized Cost 380,976 81,748
Unrealized Gains 1,506 8
Unrealized Losses (1,874) (1,853)
Fair Value 380,608 79,903
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Fair Value of Investments, Less than 12 months 111,207 42,267
Unrealized Losses, Less than 12 months (896) (877)
Fair Value of Investments, 12 months or longer 29,606 30,586
Unrealized Losses, 12 months or longer (978) (976)
Fair Value of Investments 140,813 72,853
Unrealized Losses $ (1,874) $ (1,853)