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Acquisitions (Land Parcels) (Details)
$ in Thousands
1 Months Ended
Aug. 31, 2017
USD ($)
a
Jul. 31, 2017
USD ($)
a
Sep. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Property, Plant and Equipment [Line Items]        
Construction in progress     $ 429,390 $ 365,960
Ashburn, Virginia And Phoenix, Arizona Land | Construction in Progress        
Property, Plant and Equipment [Line Items]        
Fair value of land acquired     $ 42,000  
Land in Ashburn, Virginia        
Property, Plant and Equipment [Line Items]        
Acres of land | a 24      
Payments to Acquire Land $ 17,000      
Phoenix Arizona Land        
Property, Plant and Equipment [Line Items]        
Acres of land | a   84    
Payments to Acquire Land   $ 25,000