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Investments (Unrealized Losses) (Details) - USD ($)
Sep. 30, 2018
Dec. 31, 2017
Schedule of Investments [Line Items]    
Unrealized losses related to marketability of securities or issuer's ability to honor redemption obligations $ 0
Investment securities available for sale:    
Fair Value, Less than 12 months 2,846,381,000 3,752,642,000
Unrealized Losses, Less than 12 months 29,397,000 21,458,000
Fair Value, 12 months or more 1,516,296,000 2,975,631,000
Unrealized Losses, Greater than 12 months 60,586,000 59,470,000
Fair Value, Total 4,362,677,000 6,728,273,000
Unrealized Losses, Total 89,983,000 80,928,000
U. S. Treasury    
Investment securities available for sale:    
Fair Value, Less than 12 months 1,495,438,000 1,408,166,000
Unrealized Losses, Less than 12 months 3,996,000 345,000
Fair Value, 12 months or more 9,996,000 249,698,000
Unrealized Losses, Greater than 12 months 2,000 201,000
Fair Value, Total 1,505,434,000 1,657,864,000
Unrealized Losses, Total 3,998,000 546,000
Government Agency    
Investment securities available for sale:    
Fair Value, Less than 12 months 103,883,000 848,000
Unrealized Losses, Less than 12 months 432,000 12,000
Fair Value, 12 months or more 2,707,000 2,527,000
Unrealized Losses, Greater than 12 months 42,000 28,000
Fair Value, Total 106,590,000 3,375,000
Unrealized Losses, Total 474,000 40,000
Mortgage Backed Securities    
Investment securities available for sale:    
Fair Value, Less than 12 months 1,228,345,000 2,333,254,000
Unrealized Losses, Less than 12 months 24,796,000 20,911,000
Fair Value, 12 months or more 1,498,593,000 2,723,406,000
Unrealized Losses, Greater than 12 months 60,518,000 59,241,000
Fair Value, Total 2,726,938,000 5,056,660,000
Unrealized Losses, Total 85,314,000 80,152,000
Investment securities held to maturity    
Fair Value, Less than 12 months 2,124,316,000  
Unrealized Losses, Less than 12 months 14,907,000  
Fair Value, 12 months or more 10,549,000  
Unrealized Losses, 12 months or more 286,000  
Fair Value, Total 2,134,865,000  
Unrealized Losses, Total 15,193,000 0
Corporate bonds    
Investment securities available for sale:    
Fair Value, Less than 12 months   5,025,000
Unrealized Losses, Less than 12 months   8,000
Fair Value, 12 months or more   0
Unrealized Losses, Greater than 12 months   0
Fair Value, Total   5,025,000
Unrealized Losses, Total   8,000
Equity Securities    
Investment securities available for sale:    
Unrealized Losses, Total   0
Corporate Bonds    
Investment securities available for sale:    
Fair Value, Less than 12 months 18,715,000  
Unrealized Losses, Less than 12 months 173,000  
Fair Value, 12 months or more 5,000,000  
Unrealized Losses, Greater than 12 months 24,000  
Fair Value, Total 23,715,000  
Unrealized Losses, Total 197,000 8,000
Other    
Investment securities available for sale:    
Fair Value, Less than 12 months   5,349,000
Unrealized Losses, Less than 12 months   182,000
Fair Value, 12 months or more   0
Unrealized Losses, Greater than 12 months   0
Fair Value, Total   5,349,000
Unrealized Losses, Total $ 0 $ 182,000