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Share Incentive and Other Compensation - Employee Share Purchase Plan and Deferred Share Plan - Additional Information (Details) - USD ($)
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee share purchase discount rate 15.00%  
Employee share purchase maximum purchase amount $ 25,000  
Common Shares    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee share purchase (in shares) 2,836 3,392