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Share-Based Compensation - Other Award Types (Details) - Other Award Types - USD ($)
$ 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]      
Share-based liabilities paid $ 1.5 $ 0.8  
Number of shares that became available under the Plan 45,781 95,210 11,618
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 4 years