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Investment Securities (Fair Value and Gross Unrealized Loss) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2019
Available for sale:    
Fair value, less than 12 months   $ 523,797
Marketable securities, less than 12 months $ 221,140  
Unrealized losses, less than 12 Months   (2,274)
Marketable securities, less than 12 Months (2,223)  
Fair value, 12 months or more   1,448,109
Marketable securities, 12 months or more 2,537,693  
Unrealized losses, 12 Months or More   (14,340)
Marketable securities, 12 Months or More (77,051)  
Fair value, Total   1,971,906
Marketable securities, Total 2,758,833  
Unrealized losses, Total   (16,614)
Marketable securities, Total 79,274  
Residential mortgage-backed securities    
Available for sale:    
Fair value, less than 12 months 208,384 523,031
Unrealized losses, less than 12 Months (2,124) (2,269)
Fair value, 12 months or more 2,537,181 1,448,109
Unrealized losses, 12 Months or More (77,045) (14,340)
Fair value, Total 2,745,565 1,971,140
Unrealized losses, Total (79,169) (16,609)
Obligations of states and political subdivisions    
Available for sale:    
Fair value, less than 12 months 12,756 766
Unrealized losses, less than 12 Months (99) (5)
Fair value, 12 months or more 512  
Unrealized losses, 12 Months or More (6)  
Fair value, Total 13,268 766
Unrealized losses, Total $ (105) $ (5)