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Debt - Unsecured Revolving Credit Facility (Details) - Unsecured Revolving Credit Facility - Unsecured term loan facility
6 Months Ended
Jun. 30, 2017
USD ($)
option
Line of Credit Facility [Line Items]  
Current borrowing capacity $ 1,100,000,000.0
Maximum borrowing capacity $ 1,250,000,000.00
Number of options to extend maturity | option 2
Extension period 6 months
Extension fee, as a percent of outstanding commitments under revolving credit facility 0.075%