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Notes Payable - Maturities (Details) - USD ($)
$ in Thousands
1 Months Ended 12 Months Ended
Jan. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Maturity of notes and bonds payable        
Principal payments on notes payable   $ 725,000 $ 275,000 $ 150,000
Notes and bonds payable        
Maturity of notes and bonds payable        
2018   350,000    
2021   250,000    
2022   950,000    
Thereafter   3,700,000    
Totals   $ 5,250,000    
Weighted average interest rate   3.90%    
Weighted average remaining years until maturity   9 years 3 months 18 days    
Interest incurred on notes and bonds   $ 197,100 $ 171,500 $ 179,500
Notes and bonds payable | Minimum        
Maturity of notes and bonds payable        
Debt service coverage ratio   1.5    
Total unencumbered assets as a percentage of outstanding unsecured debt   150.00%    
Notes and bonds payable | Maximum        
Maturity of notes and bonds payable        
Debt to total adjusted assets ratio (as a percent)   60.00%    
Secured debt to total adjusted assets ratio (as a percent)   40.00%    
Notes Payable | 2.000% notes, issued in October 2012 and due in January 2018        
Maturity of notes and bonds payable        
Interest rate (as a percent)   2.00% 2.00%  
Notes Payable | 2.000% notes, issued in October 2012 and due in January 2018 | Subsequent event        
Maturity of notes and bonds payable        
Principal payments on notes payable $ 350,000      
Interest rate (as a percent) 2.00%