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Equity - Stock Based Compensation (Details)
$ in Thousands
3 Months Ended 12 Months Ended
Jul. 06, 2011
USD ($)
shares
Mar. 31, 2019
USD ($)
item
shares
Mar. 31, 2018
USD ($)
Dec. 31, 2017
shares
Dec. 31, 2016
shares
Dec. 31, 2015
shares
Restricted stock            
Stock-based incentive plan awards            
Compensation expense, net of capitalization   $ 2,700 $ 2,300      
LTIP Retention Award to Chairman and CEO            
Stock-based incentive plan awards            
Awards earned (in units) | shares 1,000,000          
Grant Date Fair Value $ 120,300          
Service period 8 years          
LTIP Retention Award to Chairman and CEO | A Units            
Stock-based incentive plan awards            
Awards earned (in units) | shares           360,000
LTIP Retention Award to Chairman and CEO | B Units            
Stock-based incentive plan awards            
Awards earned (in units) | shares         360,000  
LTIP Retention Award to Chairman and CEO | C Units            
Stock-based incentive plan awards            
Awards earned (in units) | shares       280,000    
LTIP programs            
Stock-based incentive plan awards            
Percent of distributions of Operating Partnership that participants are entitled to receive during performance period   10.00%        
Compensation expense, net of capitalization   $ 3,900 $ 4,800      
2014-2016 LTIP program | LTIP Units            
Stock-based incentive plan awards            
LTIP Units Earned (in units) | shares   120,314        
Grant Date Fair Value   $ 27,500        
2015-2017 LTIP program | LTIP Units            
Stock-based incentive plan awards            
Grant Date Fair Value   21,600        
2016-2018 LTIP program | LTIP Units            
Stock-based incentive plan awards            
Grant Date Fair Value   $ 22,700        
2018 LTIP program | LTIP Units            
Stock-based incentive plan awards            
Number of tranches | item   2        
Grant Date Fair Value   $ 12,100        
2018 LTIP program | LTIP Units | Maximum            
Stock-based incentive plan awards            
Grant Date Target Value   $ 18,200        
2018 LTIP program | LTIP Units | Tranche A            
Stock-based incentive plan awards            
Vesting rights percentage   50.00%        
Performance period   2 years        
Grant Date Fair Value   $ 6,100        
Grant Date Target Value   $ 6,100        
2018 LTIP program | LTIP Units | Tranche B            
Stock-based incentive plan awards            
Performance period   3 years        
Grant Date Fair Value   $ 6,100        
Grant Date Target Value   $ 6,100