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Stock Repurchase Program (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Oct. 31, 2018
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Mar. 31, 2018
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)   217,388 217,388          
Stock repurchased during period, value   $ 2.9 $ 2.9          
Remaining number of shares authorized to be repurchased (in shares)   282,612 282,612          
Subsequent Event | March 2018 Plan                
Class of Stock [Line Items]                
Shares repurchase and retired during period (in shares) 133,344              
Stock repurchased during period, value $ 1.0