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Stock-based Compensation - Restricted Share Units (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2020
Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Non-cash share compensation       $ 2,996 $ 2,653
2013 Plan | Restricted share units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Non-cash share compensation       2,100 $ 1,800
Unrecognized compensation expense $ 15,300     $ 15,300  
2013 Plan | Trustee Awards          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted (in shares)       38,858  
Granted (in dollars per share)       $ 19.30  
2013 Plan | Performance based restricted stock units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted (in dollars per share) $ 24.36        
Performance conditions grant date fair value (in dollars per share) $ 19.30        
Officers and Certain Employees | 2013 Plan | Restricted share units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted (in shares) 482,646        
Vesting period     3 years    
Officers and Certain Employees | 2013 Plan | Market Based Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting percentage 40.00%        
Officers and Certain Employees | 2013 Plan | Performance based restricted stock units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting percentage 60.00%        
Percentage of restricted share units issued to trustees   100.00%      
Officer | 2013 Plan | Restricted share units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period     5 years    
Trustees | 2013 Plan | Restricted share units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period     2 years    
Trustees | 2013 Plan | Trustee Awards          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted (in shares) 38,858        
Director | 2013 Plan | Restricted share units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of dividend equivalent included in each award (in shares) 1     1  
Minimum | 2013 Plan | Market Based Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted (in dollars per share) $ 26.80        
Maximum | 2013 Plan | Market Based Restricted Stock Units (RSUs)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted (in dollars per share) $ 37.09