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Share Incentive and Other Compensation - Additional Information (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2009
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Unit based compensation $ 3,454,000 $ 3,809,000    
Trustee fees 300,000 300,000    
Total unrecognized compensation cost related to nonvested awards $ 21,500,000      
Weighted-average period over which cost is expected to be recognized 1 year 10 months 24 days      
Performance Period Ending December 31, 2021        
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 $ 8,400,000   $ 12,800,000  
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      
LTIP Units | Performance Period Ending December 31, 2021        
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 $ 200,000   $ 800,000  
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      
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 $ 11,100,000      
Weighted average grant date fair value, grants (in dollars per share) $ 33.04   $ 26.64  
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 $ 1,900,000 2,200,000    
LTIP Units and Restricted Stock | Performance Period Ending December 31, 2021        
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 $ 100,000 100,000    
Share Incentive Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares authorized 731,267      
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 330,718      
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 8,041      
Performance Shares 2018        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Performance period 3 years      
Volatility rate (19.60%)      
Risk-free interest rates (2.50%)      
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 0    
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      
Compensation expense 0 0    
Long Term Investment Alignment Program | Fund V        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Percentage of promote awarded as share based compensation award       2.20%
Awards in connection with fund to have intrinsic value 0      
Compensation expense $ 0 $ 0