Acquisitions (Land Acquisitions) (Details) $ in Millions	1 Months Ended			12 Months Ended
	Oct. 31, 2017 USD ($) a	Aug. 31, 2017 USD ($) a	Jul. 31, 2017 USD ($) a	Dec. 31, 2017 USD ($) item
Property, Plant and Equipment [Line Items]
Number of acquisitions | item				4
Land in Ashburn, Virginia
Property, Plant and Equipment [Line Items]
Acres of land | a	28	24
Payments to Acquire Land	$ 36.0	$ 17.0
Phoenix Arizona Land
Property, Plant and Equipment [Line Items]
Acres of land | a			84
Payments to Acquire Land			$ 25.0
Land in Hillsboro, Oregon
Property, Plant and Equipment [Line Items]
Acres of land | a	92
Payments to Acquire Land	$ 26.0
Phoenix Arizona And Ashburn Virginia And Hillsboro Oregon [Member] | Construction in Progress
Property, Plant and Equipment [Line Items]
Fair value of land acquired				$ 163.6