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Summary of Significant Accounting Policies - Share-based Compensation (Details) - Performance shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares awarded as a percentage of granted   130.20% 131.50%
Vesting period 3 years    
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares awarded as a percentage of granted 0.00%    
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares awarded as a percentage of granted 200.00%