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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, 2019
Mar. 31, 2018
Mar. 31, 2019
Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Non-cash share compensation     $ 2,653 $ 2,605
2013 Plan | Restricted share units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Non-cash share compensation     1,800 $ 1,800
Unrecognized compensation expense $ 9,900   $ 9,900  
2013 Plan | Performance based restricted stock units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in dollars per share) $ 25.27      
Performance conditions grant date fair value (in dollars per share) $ 17.89      
Officers and Certain Employees | 2013 Plan | Restricted share units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in shares) 229,884      
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 50.00%      
Granted (in dollars per share) $ 29.60      
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 50.00%      
Percentage of restricted share units issued to trustees 100.00%      
Trustees | 2013 Plan | Restricted share units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in shares) 41,925      
Vesting period   2 years    
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  
Market Condition Number Two | 2013 Plan | Market Based Restricted Stock Units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in dollars per share) $ 35.70