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Investment Securities Gross Unrealized Losses and Fair Values of Investments by Category and Length of Time in a Continuous Unrealized Loss Position (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
U.S. Government sponsored agency securities    
Estimated Fair Value, Less than 12 Months $ 5,830 $ 247,509
Unrealized Losses, Less than 12 Months (26) (15,676)
Estimated Fair Value, 12 Months or Longer 0 0
Unrealized Losses, 12 Months or Longer 0 0
Estimated Fair Value, Total 5,830 247,509
Unrealized Losses, Total (26) (15,676)
State and municipal securities    
Estimated Fair Value, Less than 12 Months 11,650  
Unrealized Losses, Less than 12 Months (50)  
Estimated Fair Value, 12 Months or Longer 118,297  
Unrealized Losses, 12 Months or Longer (2,499)  
Estimated Fair Value, Total 129,947  
Unrealized Losses, Total (2,549)  
Corporate debt securities    
Estimated Fair Value, Less than 12 Months 4,544 11,922
Unrealized Losses, Less than 12 Months (48) (110)
Estimated Fair Value, 12 Months or Longer 32,163 34,629
Unrealized Losses, 12 Months or Longer (1,828) (4,475)
Estimated Fair Value, Total 36,707 46,551
Unrealized Losses, Total (1,876) (4,585)
Collateralized mortgage obligations    
Estimated Fair Value, Less than 12 Months 303,932 166,905
Unrealized Losses, Less than 12 Months (2,408) (3,899)
Estimated Fair Value, 12 Months or Longer 187,690 258,237
Unrealized Losses, 12 Months or Longer (7,387) (8,279)
Estimated Fair Value, Total 491,622 425,142
Unrealized Losses, Total (9,795) (12,178)
Mortgage-backed securities    
Estimated Fair Value, Less than 12 Months   1,137,510
Unrealized Losses, Less than 12 Months   (17,437)
Estimated Fair Value, 12 Months or Longer   0
Unrealized Losses, 12 Months or Longer   0
Estimated Fair Value, Total   1,137,510
Unrealized Losses, Total   (17,437)
Residential mortgage-backed securities    
Estimated Fair Value, Less than 12 Months 511,378  
Unrealized Losses, Less than 12 Months (4,348)  
Estimated Fair Value, 12 Months or Longer 500,375  
Unrealized Losses, 12 Months or Longer (10,893)  
Estimated Fair Value, Total 1,011,753  
Unrealized Losses, Total (15,241)  
Residential mortgage-backed securities    
Estimated Fair Value, Less than 12 Months 190,985  
Unrealized Losses, Less than 12 Months (2,118)  
Estimated Fair Value, 12 Months or Longer 21,770  
Unrealized Losses, 12 Months or Longer (478)  
Estimated Fair Value, Total 212,755  
Unrealized Losses, Total (2,596)  
Auction rate securities    
Estimated Fair Value, Less than 12 Months 0 0
Unrealized Losses, Less than 12 Months 0 0
Estimated Fair Value, 12 Months or Longer 98,668 97,256
Unrealized Losses, 12 Months or Longer (8,742) (9,959)
Estimated Fair Value, Total 98,668 97,256
Unrealized Losses, Total (8,742) (9,959)
Debt securities    
Estimated Fair Value, Less than 12 Months 1,028,319 1,563,846
Unrealized Losses, Less than 12 Months (8,998) (37,122)
Estimated Fair Value, 12 Months or Longer 958,963 390,122
Unrealized Losses, 12 Months or Longer (31,827) (22,713)
Estimated Fair Value, Total 1,987,282 1,953,968
Unrealized Losses, Total $ (40,825) $ (59,835)