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Equity - Stock Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
9 Months Ended 12 Months Ended
Jul. 06, 2011
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted stock            
Stock-based incentive plan awards            
Compensation expense, net of capitalization   $ 8.3 $ 6.0      
Restricted stock | Employees            
Stock-based incentive plan awards            
Vesting service period   3 years        
Awards earned (in units)   84,503        
Weighted average fair value of shares granted during the period (in dollars per share)   $ 181.85        
Restricted stock | Non-employee Director            
Stock-based incentive plan awards            
Vesting service period   1 year        
Awards earned (in units)   10,644        
Weighted average fair value of shares granted during the period (in dollars per share)   $ 174.03        
LTIP Retention Award to Chairman and CEO            
Stock-based incentive plan awards            
Awards earned (in units) 1,000,000          
Grant Date Fair Value $ 120.3          
Service period 8 years          
LTIP Retention Award to Chairman and CEO | A Units            
Stock-based incentive plan awards            
Awards earned (in units)           360,000
LTIP Retention Award to Chairman and CEO | B Units            
Stock-based incentive plan awards            
Awards earned (in units)         360,000  
LTIP Retention Award to Chairman and CEO | C Units            
Stock-based incentive plan awards            
Awards earned (in units)       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   $ 12.2 $ 13.0      
2014-2016 LTIP program | LTIP Units            
Stock-based incentive plan awards            
LTIP Units Earned (in units)   120,314        
Grant Date Fair Value   $ 27.5        
2015-2017 LTIP program | LTIP Units            
Stock-based incentive plan awards            
Grant Date Fair Value   21.6        
2016-2018 LTIP program | LTIP Units            
Stock-based incentive plan awards            
Grant Date Fair Value   22.7        
2018 LTIP program | LTIP Units | Tranche A            
Stock-based incentive plan awards            
Grant Date Fair Value   6.1        
Grant Date Target Value   6.1        
2018 LTIP program | LTIP Units | Tranche B            
Stock-based incentive plan awards            
Grant Date Fair Value   6.1        
Grant Date Target Value   $ 6.1        
2019 LTIP program | LTIP Units            
Stock-based incentive plan awards            
Performance period   3 years        
Grant Date Fair Value   $ 9.5        
Grant Date Target Value   14.7        
2019 LTIP program | LTIP Units | Maximum            
Stock-based incentive plan awards            
Grant Date Target Value   $ 22.1