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Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Details)
$ in Thousands
Mar. 31, 2017
USD ($)
Investment
Dec. 31, 2016
USD ($)
Mar. 31, 2016
Investment
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost $ 8,615,695 $ 8,426,998  
Unrealized Gains 14,996 16,968  
Unrealized Losses (62,874) (67,828)  
Fair Value 8,567,817 8,376,138  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 6,254,576 5,943,190  
Unrealized Losses, Less than 12 months (55,788) (60,189)  
Fair Value of Investments, 12 months or longer 286,127 295,657  
Unrealized Losses, 12 months or longer (7,086) (7,639)  
Fair Value of Investments 6,540,703 6,238,847  
Unrealized Losses $ (62,874) (67,828)  
Number of held-to-maturity investments with unrealized loss | Investment 478   462
Number of held-to-maturity investments in continuous loss more than 12 months | Investment 96   85
U.S. agency debentures      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost $ 641,421 622,445  
Unrealized Gains 7,106 7,840  
Unrealized Losses (1,948) (1,198)  
Fair Value 646,579 629,087  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 117,916 118,721  
Unrealized Losses, Less than 12 months (1,948) (1,198)  
Fair Value of Investments, 12 months or longer 0 0  
Unrealized Losses, 12 months or longer 0 0  
Fair Value of Investments 117,916 118,721  
Unrealized Losses (1,948) (1,198)  
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost 3,020,929 2,896,179  
Unrealized Gains 5,498 6,919  
Unrealized Losses (16,694) (24,526)  
Fair Value 3,009,733 2,878,572  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 2,152,438 1,801,861  
Unrealized Losses, Less than 12 months (15,863) (23,558)  
Fair Value of Investments, 12 months or longer 21,350 21,917  
Unrealized Losses, 12 months or longer (831) (968)  
Fair Value of Investments 2,173,788 1,823,778  
Unrealized Losses (16,694) (24,526)  
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost 3,223,055 3,362,598  
Unrealized Gains 638 788  
Unrealized Losses (30,595) (31,274)  
Fair Value 3,193,098 3,332,112  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 2,749,432 2,729,889  
Unrealized Losses, Less than 12 months (25,442) (25,723)  
Fair Value of Investments, 12 months or longer 213,674 228,220  
Unrealized Losses, 12 months or longer (5,153) (5,551)  
Fair Value of Investments 2,963,106 2,958,109  
Unrealized Losses (30,595) (31,274)  
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost 299,257 312,665  
Unrealized Gains 217 176  
Unrealized Losses (477) (1,339)  
Fair Value 298,997 311,502  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 89,927 251,012  
Unrealized Losses, Less than 12 months (469) (1,339)  
Fair Value of Investments, 12 months or longer 3,234 0  
Unrealized Losses, 12 months or longer (8) 0  
Fair Value of Investments 93,161 251,012  
Unrealized Losses (477) (1,339)  
Agency-issued commercial mortgage-backed securities      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost 1,292,744 1,151,363  
Unrealized Gains 1,242 1,237  
Unrealized Losses (11,558) (7,638)  
Fair Value 1,282,428 1,144,962  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 1,106,777 999,440  
Unrealized Losses, Less than 12 months (11,453) (7,494)  
Fair Value of Investments, 12 months or longer 14,225 14,934  
Unrealized Losses, 12 months or longer (105) (144)  
Fair Value of Investments 1,121,002 1,014,374  
Unrealized Losses (11,558) (7,638)  
US States and Political Subdivisions Debt Securities [Member]      
Schedule of Held-to-maturity Securities [Line Items]      
Amortized Cost 138,289 81,748  
Unrealized Gains 295 8  
Unrealized Losses (1,602) (1,853)  
Fair Value 136,982 79,903  
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]      
Fair Value of Investments, Less than 12 months 38,086 42,267  
Unrealized Losses, Less than 12 months (613) (877)  
Fair Value of Investments, 12 months or longer 33,644 30,586  
Unrealized Losses, 12 months or longer (989) (976)  
Fair Value of Investments 71,730 72,853  
Unrealized Losses $ (1,602) $ (1,853)