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Investment Securities (Fair Value and Gross Unrealized Loss) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2020
Available for sale:    
Fair value, less than 12 months   $ 1,462,232
Marketable securities, less than 12 months $ 523,797  
Unrealized losses, less than 12 Months   (9,339)
Marketable securities, less than 12 Months (2,274)  
Fair value, 12 months or more   757
Marketable securities, 12 months or more 1,448,109  
Unrealized losses, 12 Months or More   (14)
Marketable securities, 12 Months or More (14,340)  
Fair value, Total   1,462,989
Marketable securities, Total 1,971,906  
Unrealized losses, Total   (9,353)
Marketable securities, Total 16,614  
Collateralized Mortgage Backed Securities [Member]    
Available for sale:    
Fair value, less than 12 months 523,031 1,462,232
Unrealized losses, less than 12 Months (2,269) (9,339)
Fair value, 12 months or more 1,448,109  
Unrealized losses, 12 Months or More (14,340)  
Fair value, Total 1,971,140 1,462,232
Unrealized losses, Total (16,609) (9,339)
US States and Political Subdivisions Debt Securities [Member]    
Available for sale:    
Fair value, less than 12 months 766  
Unrealized losses, less than 12 Months (5)  
Fair value, 12 months or more   757
Unrealized losses, 12 Months or More   (14)
Fair value, Total 766 757
Unrealized losses, Total $ (5) $ (14)