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Stock Repurchase Program (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 31, 2018
Feb. 28, 2017
Feb. 29, 2016
Dec. 31, 2014
Dec. 31, 2012
July 2012 Plan                
Class of Stock                
Stock repurchased program authorized number of shares (in shares)               1,000,000
January 2014 Plan                
Class of Stock                
Stock repurchased program authorized number of shares (in shares)             500,000  
2012 And 2014 Plan                
Class of Stock                
Stock repurchase program, remaining number of shares authorized to be repurchased (in shares)     56,000       268,000  
2012 And 2014 Plan | Treasury Stock                
Class of Stock                
Stock repurchased during period (in shares)     212,000          
Aggregate purchase price of shares repurchased     $ 1.7          
February 2016 Plan                
Class of Stock                
Stock repurchased program authorized number of shares (in shares)           1,000,000    
February 2016 Plan | Treasury Stock                
Class of Stock                
Stock repurchased during period (in shares)   1,056,000            
Aggregate purchase price of shares repurchased   $ 9.5            
February 2017 Plan                
Class of Stock                
Stock repurchased program authorized number of shares (in shares)         1,000,000      
February 2017 Plan | Treasury Stock                
Class of Stock                
Stock repurchased during period (in shares) 1,000,000              
Aggregate purchase price of shares repurchased $ 9.7              
Subsequent Event | March 2018 Plan                
Class of Stock                
Stock repurchased program authorized number of shares (in shares)       500,000