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Investment Securities and Equity Securities with Readily Determinable Fair Values (Fair Value and Gross Unrealized Loss) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Sep. 30, 2018
Available for sale:    
Fair value, less than 12 months   $ 832,939
Marketable securities, less than 12 months $ 1,078,610  
Unrealized losses, less than 12 Months   (18,389)
Marketable securities, less than 12 Months (13,417)  
Fair value, 12 months or more   2,211,383
Marketable securities, 12 months or more 2,037,996  
Unrealized losses, 12 Months or More   (111,403)
Marketable securities, 12 Months or More (53,050)  
Fair value, Total   3,044,322
Marketable securities, Total 3,116,606  
Unrealized losses, Total   (129,792)
Marketable securities, Total (66,467)  
Residential mortgage-backed securities    
Available for sale:    
Fair value, less than 12 months 1,061,577 812,981
Unrealized losses, less than 12 Months (13,157) (18,128)
Fair value, 12 months or more 2,029,455 2,210,870
Unrealized losses, 12 Months or More (52,812) (111,395)
Fair value, Total 3,091,032 3,023,851
Unrealized losses, Total (65,969) (129,523)
Obligations of states and political subdivisions    
Available for sale:    
Fair value, less than 12 months 5,534 19,958
Unrealized losses, less than 12 Months (9) (261)
Fair value, 12 months or more 522 513
Unrealized losses, 12 Months or More (7) (8)
Fair value, Total 6,056 20,471
Unrealized losses, Total (16) $ (269)
Equity securities    
Available for sale:    
Equity Securities, less than 12 months 11,499  
Equity Securities, less than 12 Months (251)  
Equity Securities, 12 months or more 8,019  
Equity Securities, 12 Months or More (231)  
Equity Securities, Total 19,518  
Equity securities $ (482)