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Investment Securities (Narrative) (Details)
$ in Thousands
Mar. 31, 2018
USD ($)
position
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Investments [Abstract]      
Pledged to secure deposits $ 1,010,000 $ 2,000,000  
Investment securities in a loss position for less than twelve months | position 75    
Investment securities in a loss position for twelve months or longer | position 54    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Adjustment from adoption of ASU   (685) $ 0
ASU 2016-01      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Adjustment from adoption of ASU $ 3,200 $ 0