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Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Details)
$ in Thousands
Jun. 30, 2017
USD ($)
Investment
Dec. 31, 2016
USD ($)
Jun. 30, 2016
Investment
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost $ 9,938,371 $ 8,426,998  
Unrealized Gains 24,992 16,968  
Unrealized Losses (52,859) (67,828)  
Fair Value 9,910,504 8,376,138  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 6,231,134 5,943,190  
Unrealized Losses, Less than 12 months (46,654) (60,189)  
Fair Value of Investments, 12 months or longer 284,515 295,657  
Unrealized Losses, 12 months or longer (6,205) (7,639)  
Fair Value of Investments 6,515,649 6,238,847  
Unrealized Losses $ (52,859) (67,828)  
Number of held-to-maturity investments with unrealized loss | Investment 468   462
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 97   85
U.S. agency debentures      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost $ 641,191 622,445  
Unrealized Gains 7,553 7,840  
Unrealized Losses (1,291) (1,198)  
Fair Value 647,453 629,087  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 63,160 118,721  
Unrealized Losses, Less than 12 months (1,291) (1,198)  
Fair Value of Investments, 12 months or longer 0 0  
Unrealized Losses, 12 months or longer 0 0  
Fair Value of Investments 63,160 118,721  
Unrealized Losses (1,291) (1,198)  
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost 4,170,585 2,896,179  
Unrealized Gains 12,047 6,919  
Unrealized Losses (14,619) (24,526)  
Fair Value 4,168,013 2,878,572  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 2,242,392 1,801,861  
Unrealized Losses, Less than 12 months (13,972) (23,558)  
Fair Value of Investments, 12 months or longer 20,690 21,917  
Unrealized Losses, 12 months or longer (647) (968)  
Fair Value of Investments 2,263,082 1,823,778  
Unrealized Losses (14,619) (24,526)  
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost 3,240,643 3,362,598  
Unrealized Gains 853 788  
Unrealized Losses (26,227) (31,274)  
Fair Value 3,215,269 3,332,112  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 2,719,351 2,729,889  
Unrealized Losses, Less than 12 months (21,887) (25,723)  
Fair Value of Investments, 12 months or longer 201,313 228,220  
Unrealized Losses, 12 months or longer (4,340) (5,551)  
Fair Value of Investments 2,920,664 2,958,109  
Unrealized Losses (26,227) (31,274)  
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost 283,519 312,665  
Unrealized Gains 581 176  
Unrealized Losses (57) (1,339)  
Fair Value 284,043 311,502  
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 17,247 0  
Unrealized Losses, 12 months or longer (57) 0  
Fair Value of Investments 17,247 251,012  
Unrealized Losses (57) (1,339)  
Agency-issued commercial mortgage-backed securities      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost 1,386,455 1,151,363  
Unrealized Gains 2,507 1,237  
Unrealized Losses (9,267) (7,638)  
Fair Value 1,379,695 1,144,962  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 1,166,556 999,440  
Unrealized Losses, Less than 12 months (9,159) (7,494)  
Fair Value of Investments, 12 months or longer 13,658 14,934  
Unrealized Losses, 12 months or longer (108) (144)  
Fair Value of Investments 1,180,214 1,014,374  
Unrealized Losses (9,267) (7,638)  
US States and Political Subdivisions Debt Securities [Member]      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost 215,978 81,748  
Unrealized Gains 1,451 8  
Unrealized Losses (1,398) (1,853)  
Fair Value 216,031 79,903  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 39,675 42,267  
Unrealized Losses, Less than 12 months (345) (877)  
Fair Value of Investments, 12 months or longer 31,607 30,586  
Unrealized Losses, 12 months or longer (1,053) (976)  
Fair Value of Investments 71,282 72,853  
Unrealized Losses $ (1,398) $ (1,853)