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Shareholders' Equity / Partners' Capital - 2012 Equity Incentive Plan (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2012
Class of Stock [Line Items]        
Noncash share-based compensation $ 4,212 $ 3,636 $ 3,125  
Restatement Adjustment | Reclassification of Noncash Share-based Compensation to General and Administrative Expenses        
Class of Stock [Line Items]        
Share-based compensation expense 2,600 2,100 1,900  
Restatement Adjustment | Reclassification of Noncash Share-based Compensation to Property Management Expenses        
Class of Stock [Line Items]        
Share-based compensation expense 1,600 1,500 1,200  
Stock options and Restricted stock units        
Class of Stock [Line Items]        
Noncash share-based compensation 4,200 $ 3,600 $ 3,100  
Stock options        
Class of Stock [Line Items]        
Total unrecognized compensation cost $ 2,900      
Weighted-average period of unvested cost 2 years 2 months 19 days      
Restricted stock units        
Class of Stock [Line Items]        
Total unrecognized compensation cost $ 3,700      
Weighted-average period of unvested cost 2 years 10 months 28 days      
2012 Equity Incentive Plan | Board of Trustees        
Class of Stock [Line Items]        
Vesting period 4 years 4 years 4 years  
Expiration period 10 years 10 years 10 years  
2012 Equity Incentive Plan | Restricted stock units        
Class of Stock [Line Items]        
Stock options granted (in shares) 174,400 74,100 44,000  
2012 Equity Incentive Plan | Class A common shares/units        
Class of Stock [Line Items]        
Shares available for issuance (in shares)       6,000,000
2012 Equity Incentive Plan | Class A common shares/units | Stock options        
Class of Stock [Line Items]        
Stock options granted (in shares) 385,200 708,000 588,500