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Debt - Unsecured Revolving Credit and Term Loan Facility (Details)
1 Months Ended 3 Months Ended 9 Months Ended
Aug. 31, 2017
USD ($)
option
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Line of Credit Facility [Line Items]            
Loss on early extinguishment of debt   $ 2,157,000 $ 0 $ 2,157,000 $ 552,000  
Unsecured facility   0   0   $ 0
Unsecured Revolving Credit Facility and Term Loan | Revolving Credit Facility            
Line of Credit Facility [Line Items]            
Maximum borrowing capacity $ 1,365,000,000.000          
Unsecured revolving credit facility higher borrowing capacity option 1,750,000,000.00          
Unsecured revolving credit facility | Revolving Credit Facility            
Line of Credit Facility [Line Items]            
Current borrowing capacity $ 1,100,000,000.0          
Unsecured facility   0   0   0
Number of options to extend maturity | option 2          
Extension period 6 months          
Unsecured term loan facility | Revolving Credit Facility            
Line of Credit Facility [Line Items]            
Current borrowing capacity $ 265,000,000          
Unsecured facility $ 265,000,000 $ 265,000,000   $ 265,000,000   $ 265,000,000
Minimum | Unsecured revolving credit facility | Revolving Credit Facility            
Line of Credit Facility [Line Items]            
Extension fee, as a percent of outstanding commitments under revolving credit facility 0.0625%          
Maximum | Unsecured revolving credit facility | Revolving Credit Facility            
Line of Credit Facility [Line Items]            
Extension fee, as a percent of outstanding commitments under revolving credit facility 0.075%