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Share-Based Compensation - Unrecognized Compensation Expense for Share-Based Awards (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Unrecognized compensation expense $ 40.4
Remaining weighted-average recognition period 2 years 7 months 6 days
Restricted Stock  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Unrecognized compensation expense $ 30.6
Remaining weighted-average recognition period 2 years 6 months
Performance Units  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Unrecognized compensation expense $ 9.8
Remaining weighted-average recognition period 2 years 9 months 18 days