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