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Equity - Stock Based Compensation (Details)
$ / shares in Units, $ in Thousands
9 Months Ended 12 Months Ended
Jul. 06, 2011
USD ($)
shares
Sep. 30, 2018
USD ($)
item
$ / shares
shares
Sep. 30, 2017
USD ($)
Dec. 31, 2017
shares
Dec. 31, 2016
shares
Dec. 31, 2015
shares
Dec. 31, 2013
shares
Restricted stock              
Stock-based incentive plan awards              
Compensation expense, net of capitalization   $ 6,000 $ 7,600        
Restricted stock | Employees              
Stock-based incentive plan awards              
Vesting period   3 years          
Awards earned (in units) | shares   43,043          
Weighted average fair value of shares granted during the period (in dollars per share) | $ / shares   $ 154.35          
Restricted stock | Non-employee Directors              
Stock-based incentive plan awards              
Vesting period   1 year          
Awards earned (in units) | shares   12,648          
Weighted average fair value of shares granted during the period (in dollars per share) | $ / shares   $ 158.71          
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 | A Units | Maximum              
Stock-based incentive plan awards              
Shares reserved for issuance (in shares) | 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 | B Units | Maximum              
Stock-based incentive plan awards              
Shares reserved for issuance (in shares) | 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 Retention Award to Chairman and CEO | C Units | Maximum              
Stock-based incentive plan awards              
Shares reserved for issuance (in shares) | 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%          
Vesting period   2 years          
Vesting rights percentage   50.00%          
Compensation expense, net of capitalization   $ 13,000 $ 9,700        
2013-2015 LTIP program | LTIP Units              
Stock-based incentive plan awards              
LTIP Units Earned (in units) | shares   466,405          
Grant Date Fair Value   $ 28,500          
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   25,100          
2016-2018 LTIP program | LTIP Units              
Stock-based incentive plan awards              
Grant Date Fair Value   $ 26,300          
2018 LTIP program | LTIP Units              
Stock-based incentive plan awards              
Number of tranches | item   2          
Grant Date Fair Value   $ 12,500          
2018 LTIP program | LTIP Units | Maximum              
Stock-based incentive plan awards              
Grant Date Target Value   $ 18,750          
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   $ 6,250          
Grant Date Target Value   $ 6,250          
2018 LTIP program | LTIP Units | Tranche B              
Stock-based incentive plan awards              
Performance period   3 years          
Grant Date Fair Value   $ 6,250          
Grant Date Target Value   $ 6,250