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Stock Repurchase Program (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Feb. 28, 2017
Feb. 29, 2016
January 2014 and February 2016 Plan          
Class of Stock [Line Items]          
Remaining number of shares authorized to be repurchased (in shares)     0    
January 2014 and February 2016 Plan | Treasury Stock          
Class of Stock [Line Items]          
Shares repurchase and retired during period (in shares)     1,056,000    
February 2016 Plan          
Class of Stock [Line Items]          
Number of shares authorized to be repurchased (up to) (in shares)         1,000,000
February 2016 Plan | Treasury Stock          
Class of Stock [Line Items]          
Stock repurchased during period, value     $ 9.5    
January 2014 Plan          
Class of Stock [Line Items]          
Shares repurchase and retired during period (in shares)     56,000    
February 2017 Plan          
Class of Stock [Line Items]          
Number of shares authorized to be repurchased (up to) (in shares)       1,000,000  
Shares repurchase and retired during period (in shares)   1,000,000      
February 2017 Plan | Treasury Stock          
Class of Stock [Line Items]          
Stock repurchased during period, value   $ 9.7      
March 2018 Plan          
Class of Stock [Line Items]          
Number of shares authorized to be repurchased (up to) (in shares) 500,000        
Shares repurchase and retired during period (in shares) 0