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Mortgages and Notes Payable (Details)
3 Months Ended 9 Months Ended
Mar. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
extension
Oct. 16, 2018
USD ($)
Jun. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]          
Mortgages and notes payable   $ 2,087,421,000     $ 2,014,333,000
Unamortized debt issuance costs   (9,654,000)     (8,161,000)
Revolving Credit Facility [Member]          
Debt Instrument [Line Items]          
Maximum borrowing capacity on revolving credit facility   $ 600,000,000      
Maturity date on revolving credit facility   Jan. 01, 2022      
Additional borrowing capacity on revolving credit facility   $ 400,000,000      
Number of additional extensions | extension   2      
Term of optional extension   6 months      
Annual facility fee (in hundredths)   0.20%      
Amount outstanding on revolving credit facility   $ 184,000,000      
Outstanding letters of credit on revolving credit facility   400,000      
Unused borrowing capacity on revolving credit facility   415,600,000      
7.5% Notes Due 2018 [Member]          
Debt Instrument [Line Items]          
Principal debt amount       $ 200,000,000  
Interest rate (in hundredths)       7.50%  
4.125% (4.271% effective rate) Notes due 2028 [Member]          
Debt Instrument [Line Items]          
Principal debt amount $ 350,000,000        
Interest rate (in hundredths) 4.125%        
Scheduled maturity date Mar. 15, 2028        
Original issue discount $ 4,100,000        
Effective interest rate (in hundredths) 4.271%        
Debt issuance costs incurred $ 2,900,000        
Secured indebtedness [Member]          
Debt Instrument [Line Items]          
Mortgages and notes payable   97,636,000     98,981,000
Aggregate undepreciated book value of secured real estate assets   147,600,000      
Unsecured indebtedness [Member]          
Debt Instrument [Line Items]          
Mortgages and notes payable   $ 1,999,439,000     $ 1,923,513,000
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member]          
Debt Instrument [Line Items]          
Facility interest rate basis   LIBOR plus 100 basis points      
Interest rate, basis spread (in hundredths)   1.00%      
Subsequent Event [Member] | Revolving Credit Facility [Member]          
Debt Instrument [Line Items]          
Amount outstanding on revolving credit facility     $ 173,000,000    
Outstanding letters of credit on revolving credit facility     400,000    
Unused borrowing capacity on revolving credit facility     $ 426,600,000