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Investments (Unrealized Losses) (Details) - USD ($)
Jun. 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 $ 0
Investment securities available for sale:    
Fair Value, Less than 12 months 3,535,984,000 3,752,642,000
Unrealized Losses, Less than 12 months 55,802,000 21,458,000
Fair Value, 12 months or more 661,241,000 2,975,631,000
Unrealized Losses, Greater than 12 months 21,136,000 59,470,000
Fair Value, Total 4,197,225,000 6,728,273,000
Unrealized Losses, Total 76,938,000 80,928,000
U. S. Treasury    
Investment securities available for sale:    
Fair Value, Less than 12 months 1,503,974,000 1,408,166,000
Unrealized Losses, Less than 12 months 4,461,000 345,000
Fair Value, 12 months or more 0 249,698,000
Unrealized Losses, Greater than 12 months 0 201,000
Fair Value, Total 1,503,974,000 1,657,864,000
Unrealized Losses, Total 4,461,000 546,000
Government Agency    
Investment securities available for sale:    
Fair Value, Less than 12 months 83,383,000 848,000
Unrealized Losses, Less than 12 months 328,000 12,000
Fair Value, 12 months or more 2,004,000 2,527,000
Unrealized Losses, Greater than 12 months 27,000 28,000
Fair Value, Total 85,387,000 3,375,000
Unrealized Losses, Total 355,000 40,000
Mortgage Backed Securities    
Investment securities available for sale:    
Fair Value, Less than 12 months 1,923,954,000 2,333,254,000
Unrealized Losses, Less than 12 months 50,765,000 20,911,000
Fair Value, 12 months or more 654,212,000 2,723,406,000
Unrealized Losses, Greater than 12 months 21,107,000 59,241,000
Fair Value, Total 2,578,166,000 5,056,660,000
Unrealized Losses, Total 71,872,000 80,152,000
Investment securities held to maturity    
Fair Value, Less than 12 months 1,126,683,000  
Unrealized Losses, Less than 12 months 1,249,000  
Fair Value, 12 months or more 11,219,000  
Unrealized Losses, 12 months or more 210,000  
Fair Value, Total 1,137,902,000  
Unrealized Losses, Total 1,459,000 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 24,673,000  
Unrealized Losses, Less than 12 months 248,000  
Fair Value, 12 months or more 5,025,000  
Unrealized Losses, Greater than 12 months 2,000  
Fair Value, Total 29,698,000  
Unrealized Losses, Total 250,000 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