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Stock-based Compensation - Restricted Share Units Narrative (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 9 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Percentage of restricted share units issued to trustees         100.00%  
Non-cash share compensation         $ 4,976 $ 3,497
2013 Plan | Restricted share units (RSUs)            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Non-cash share compensation     $ 1,000 $ 600 2,700 $ 1,500
Unrecognized compensation expense     $ 6,000   $ 6,000  
2013 Plan | Trustee Awards, Restricted Stock Units (RSUs)            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Granted (in shares)         32,831  
Granted (in dollars per share)         $ 19.80  
2013 Plan | Performance based restricted stock units (RSUs)            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage     30.00%      
Granted (in dollars per share)     $ 29.34      
Performance conditions grant date fair value (in dollars per share)     $ 19.80      
Directors | 2013 Plan | Restricted share units (RSUs)            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period         2 years  
Number of dividend equivalent included in each award 1          
Directors | 2013 Plan | Trustee Awards, Restricted Stock Units (RSUs)            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Granted (in shares)   32,831        
Officers | 2013 Plan | Restricted share units (RSUs)            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Granted (in shares) 174,320          
Vesting period         3 years  
Officers | 2013 Plan | Market Based Restricted Stock Units (RSUs)            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage 70.00%          
Granted (in dollars per share) $ 33.43