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