Acquisitions (Land Parcels) (Details) $ in Thousands	1 Months Ended				12 Months Ended
	Oct. 31, 2018 USD ($) a	Oct. 31, 2017 USD ($) a	Aug. 31, 2017 USD ($) a	Jul. 31, 2017 USD ($) a	Dec. 31, 2018 USD ($) a property	Dec. 31, 2017 USD ($) item
Property, Plant and Equipment [Line Items]
Investment Building And Building Improvements					$ 1,917,251	$ 1,701,287
Finite Lived Intangible Assets Net					$ 95,451	109,451
Number Of Real Estate Properties | property					25
Construction in progress					$ 790,064	$ 567,819
Manassas, Virginia
Property, Plant and Equipment [Line Items]
Construction in progress					116,200
Manassas, Virginia | Assets Held For Sale
Property, Plant and Equipment [Line Items]
Construction in progress					$ 71,000
Ownership percentage					50.00%
Phoenix Arizona Land
Property, Plant and Equipment [Line Items]
Payments to Acquire Land				$ 25,000
Acres of land | a				84
Land in Ashburn, Virginia
Property, Plant and Equipment [Line Items]
Payments to Acquire Land		$ 36,000	$ 17,000
Acres of land | a		28	24
Hillsboro Oregon Land [Member]
Property, Plant and Equipment [Line Items]
Payments to Acquire Land		$ 26,000
Acres of land | a		92
Phoenix Arizona And Ashburn Virginia And Hillsboro Oregon [Member]
Property, Plant and Equipment [Line Items]
Number of acquisitions | item						4
Phoenix Arizona And Ashburn Virginia And Hillsboro Oregon [Member] | Construction in Progress
Property, Plant and Equipment [Line Items]
Fair value of land acquired						$ 163,600
Land in Atlanta Georgia
Property, Plant and Equipment [Line Items]
Payments to Acquire Land	$ 80,100
Investment Building And Building Improvements	400
Finite Lived Intangible Assets Net	$ 2,300
Acres of land | a	55
Construction in progress	$ 77,400
Land in Manassas, Virginia | Construction in Progress
Property, Plant and Equipment [Line Items]
Payments to Acquire Land					$ 37,000
Acres of land | a					118