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Share-Based Compensation - Employee Stock Purchase Plan (Details) - 2011 Employee Stock Purchase Plan - Employee Stock Option - Common Stock
12 Months Ended
Dec. 31, 2017
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares available for grant 922,972
Date Of Purchase  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Purchase price equal to lesser of fair market value percentage 95.00%
Date Of Grant  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Number of shares that became available under the Plan 1,500,000