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Stock-Based Compensation - Stock Option Activity (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock-Based Compensation      
Stock-based compensation expense $ 6.4 $ 5.0 $ 5.6
Income tax benefit $ 2.4 $ 1.9 2.1
Minimum      
Stock-Based Compensation      
Sum of age and years of service for accelerated vesting on retirement of certain stock options and restricted stock awards 75 years    
Stock Options      
Stock-Based Compensation      
Vesting period 3 years    
Shares      
Outstanding at beginning of year (in shares) 714    
Granted (in shares) 85    
Exercised (in shares) (146)    
Forfeited (in shares) (23)    
Outstanding at end of year (in shares) 630 714  
Options exercisable at end of year (in shares) 444    
Weighted-Average Exercise Price      
Outstanding at beginning of year (in dollars per share) $ 17.01    
Granted (in dollars per share) 36.25    
Exercised (in dollars per share) 14.08    
Forfeited (in dollars per share) 23.55    
Outstanding at end of year (in dollars per share) $ 20.03 $ 17.01  
Other information      
Intrinsic value of options exercised $ 3.6 $ 1.9 $ 3.9
Weighted-average remaining contractual term of options exercisable 5 years 3 months 18 days    
Aggregate intrinsic value of options exercisable $ 12.5    
Number of options that are vested and expected to vest (in shares) 600    
Weighted average exercise price of options that are vested and expected to vest (in dollars per share) $ 20.03    
Weighted-average remaining contractual term of options that are vested and expected to vest 6 years 2 months 12 days    
Aggregate intrinsic value of options that are vested and expected to vest $ 14.9