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Share Incentive and Other Compensation - Additional Information (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2009
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Unit based compensation     $ 8,332,000 $ 9,129,000    
Trustee fees     900,000 900,000    
Total unrecognized compensation cost related to nonvested awards $ 18,100,000   $ 18,100,000      
Weighted-average period over which cost is expected to be recognized     1 year 8 months 12 days      
Performance Period Ending December 31, 2020            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting percentage     33.30%      
Award vesting period     3 years      
LTIP Units            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Total fair value of shares that vested     $ 9,100,000   $ 7,300,000  
LTIP Units | Trustee            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares issued during period, share-based compensation, net of forfeitures     17,427      
Shares granted to trustees for trustee fees vesting on one year anniversary of grant date 8,949   8,949      
LTIP Units | Performance Period Ending December 31, 2020            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting percentage     66.70%      
Award vesting period     3 years      
LTIP Units | Tranche One | Trustee            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares issued during period, share-based compensation, net of forfeitures     8,478      
Restricted shares            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Total fair value of shares that vested     $ 800,000   $ 700,000  
Restricted shares | Trustee            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares issued during period, share-based compensation, net of forfeitures     17,050      
Shares granted to trustees for trustee fees vesting on one year anniversary of grant date 5,181   5,181      
Restricted shares | Tranche One | Trustee            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares issued during period, share-based compensation, net of forfeitures     11,869      
LTIP Units and Restricted Stock            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Total value of restricted shares and LTIP units as of the grant date     $ 10,400,000      
Weighted average grant date fair value, grants (in dollars per share)     $ 26.88   $ 31.69  
LTIP Units and Restricted Stock | Trustee            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting period     3 years      
Annual vesting rate of shares granted to trustees that begin vesting on the second anniversary of grant date     33.00%      
LTIP Units and Restricted Stock | General and Administrative Expense            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Unit based compensation $ 2,000,000 $ 2,100,000 $ 6,300,000 6,500,000    
LTIP Units and Restricted Stock | Performance Period Ending December 31, 2020            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting period     3 years      
LTIP Units and Restricted Stock | Performance Fails To Achieve, Earned Performance-Based Shares Vest At End Of Performance Period            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting percentage     60.00%      
LTIP Units and Restricted Stock | Performance Fails To Achieve            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting percentage     40.00%      
Award vesting period     2 years      
LTIP Units and Restricted Stock | Minimum | TSR percentile falls between the 25th percentile and the 50th percentile            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting percentage     50.00%      
LTIP Units and Restricted Stock | Minimum | TSR percentile falls between the 50th percentile and 75th percentile            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting percentage     100.00%      
LTIP Units and Restricted Stock | Maximum | TSR percentile falls between the 25th percentile and the 50th percentile            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting percentage     100.00%      
LTIP Units and Restricted Stock | Maximum | TSR percentile falls between the 50th percentile and 75th percentile            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting percentage     200.00%      
Employee Benefit Plans            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Compensation expense     $ 300,000 300,000    
Share Incentive Plan            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of shares authorized 1,198,594   1,198,594      
Share Incentive Plan | LTIP Units            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares issued during period, share-based compensation, net of forfeitures     373,886      
Share Incentive Plan | Restricted shares            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares issued during period, share-based compensation, net of forfeitures     5,387      
Long Term Investment Alignment Program            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Max percentage of future fund III promote that may be awarded to senior executives           25.00%
Long Term Investment Alignment Program | Fund III            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Percentage of promote awarded as share based compensation award           25.00%
Compensation expense     $ 0 $ 500,000    
Long Term Investment Alignment Program | Fund IV            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Percentage of promote awarded as share based compensation award           22.80%
Awards in connection with fund to have intrinsic value $ 0   $ 0