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Stock-Based Compensation - Performance Share Awards (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Performance Shares Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Award requisite performance period 3 years  
Period for share issuance after vesting 60 days  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]    
Total compensation cost not yet recognized $ 3.7 $ 4.3
Total compensation cost not yet recognized, period for recognition 1 year 6 months 7 days 1 year 10 months 24 days
Performance Shares Awards | Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Potential vesting percentage 0.00%  
Performance Shares Awards | Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Potential vesting percentage 200.00%  
Market Condition Performance Share Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected dividend yield (percent) 0.00% 0.00%
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]    
Outstanding (in shares) 133,907  
Granted (in shares) 89,338  
Vested (in shares) (17,592)  
Forfeited (in shares) (85,294)  
Outstanding (in shares) 120,359 133,907
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) $ 50.54  
Granted, weighted-average grant date fair value (in dollars per share) 48.86  
Vested, weighted-average grant date fair value (in dollars per share) 50.07  
Forfeited, weighted-average grant date fair value (in dollars per share) 45.64  
Outstanding, weighted-average grant date fair value (in dollars per share) $ 52.84 $ 50.54
Performance Condition Performance Share Awards    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]    
Outstanding (in shares) 0  
Granted (in shares) 37,649  
Vested (in shares) 0  
Forfeited (in shares) (2,142)  
Outstanding (in shares) 35,507 0
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) $ 0  
Granted, weighted-average grant date fair value (in dollars per share) 46.28  
Vested, weighted-average grant date fair value (in dollars per share) 0  
Forfeited, weighted-average grant date fair value (in dollars per share) 55.50  
Outstanding, weighted-average grant date fair value (in dollars per share) $ 45.72 $ 0