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Investments (Unrealized Losses) (Details) - USD ($)
Mar. 31, 2019
Dec. 31, 2018
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 390,651,000 705,526,000
Unrealized Losses, Less than 12 months 2,106,000 3,943,000
Fair Value, 12 months or more 2,578,426,000 2,796,627,000
Unrealized Losses, Greater than 12 months 29,217,000 52,489,000
Fair Value, Total 2,969,077,000 3,502,153,000
Unrealized Losses, Total 31,323,000 56,432,000
U. S. Treasury    
Investment securities available for sale:    
Fair Value, Less than 12 months 0 248,983,000
Unrealized Losses, Less than 12 months 0 113,000
Fair Value, 12 months or more 599,883,000 848,622,000
Unrealized Losses, Greater than 12 months 847,000 2,053,000
Fair Value, Total 599,883,000 1,097,605,000
Unrealized Losses, Total 847,000 2,166,000
Agency Securities    
Investment securities available for sale:    
Fair Value, Less than 12 months 219,395,000 115,273,000
Unrealized Losses, Less than 12 months 901,000 601,000
Fair Value, 12 months or more 12,590,000 2,310,000
Unrealized Losses, Greater than 12 months 186,000 38,000
Fair Value, Total 231,985,000 117,583,000
Unrealized Losses, Total 1,087,000 639,000
Mortgage Backed Securities    
Investment securities available for sale:    
Fair Value, Less than 12 months 114,863,000 262,204,000
Unrealized Losses, Less than 12 months 314,000 2,387,000
Fair Value, 12 months or more 1,956,003,000 1,940,695,000
Unrealized Losses, Greater than 12 months 28,115,000 50,376,000
Fair Value, Total 2,070,866,000 2,202,899,000
Unrealized Losses, Total 28,429,000 52,763,000
Investment securities held to maturity    
Fair Value, Less than 12 months 40,142,000 5,111,000
Unrealized Losses, Less than 12 months 291,000 181,000
Fair Value, 12 months or more 9,988,000 10,131,000
Unrealized Losses, 12 months or more 159,000 309,000
Fair Value, Total 50,130,000 15,242,000
Unrealized Losses, Total 450,000 490,000
Corporate bonds    
Investment securities available for sale:    
Fair Value, Less than 12 months   79,066,000
Unrealized Losses, Less than 12 months   842,000
Fair Value, 12 months or more   5,000,000
Unrealized Losses, Greater than 12 months   22,000
Fair Value, Total   84,066,000
Unrealized Losses, Total   864,000
Corporate Bonds    
Investment securities available for sale:    
Fair Value, Less than 12 months 56,393,000  
Unrealized Losses, Less than 12 months 891,000  
Fair Value, 12 months or more 9,950,000  
Unrealized Losses, Greater than 12 months 69,000  
Fair Value, Total 66,343,000  
Unrealized Losses, Total 960,000 864,000
Other    
Investment securities available for sale:    
Unrealized Losses, Total $ 0 $ 0