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Stock-Based Compensation - Restricted Stock Activity (Details) - Restricted Stock [Member] - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Period for share issuance after vesting 30 days    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Outstanding (in shares) 274,645    
Granted (in shares) 103,030 237,001  
Vested (in shares) (88,580)    
Forfeited (in shares) (22,573)    
Outstanding (in shares) 266,522 274,645  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]      
Outstanding, weighted-average grant date fair value (in dollars per share) $ 49.55    
Granted, weighted-average grant date fair value (in dollars per share) 51.68 $ 42.91 $ 75.40
Vested, weighted-average grant date fair value (in dollars per share) 47.64    
Forfeited, weighted-average grant date fair value (in dollars per share) 55.79    
Outstanding, weighted-average grant date fair value (in dollars per share) $ 50.48 $ 49.55  
Total compensation cost not yet recognized $ 7.5 $ 9.2  
Total compensation cost not yet recognized, period for recognition 2 years 2 years 6 months  
Total fair value of shares vested $ 4.3 $ 6.6 $ 9.2
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 4 years