Investments in Real Estate (Dispositions) (Details) ft² in Thousands, € in Millions, $ in Millions							3 Months Ended		12 Months Ended
	Feb. 09, 2018 USD ($) ft²	Aug. 01, 2016 USD ($)	Aug. 01, 2016 EUR (€)	Jul. 11, 2016 USD ($) ft² property	Jul. 05, 2016 USD ($) property	Jan. 21, 2016 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property	Jan. 25, 2018 USD ($) ft²	Jun. 30, 2016 property
Dispositions
Gross Proceeds									$ 91.3	$ 364.0
Gain on Sale									$ (39.9)	$ (169.8)
Number of real estate properties | property									205	187
Europe Portfolio Acquisition
Dispositions
Gain on Sale								$ (144.3)
Number of real estate properties | property					8
Consideration transferred		$ 212.0	€ 190		$ 818.9
Held-for-sale
Dispositions
Number of real estate properties | property									8
8025 North Interstate 35
Dispositions
Gross Proceeds									$ 20.2
Gain on Sale									(9.6)
44874 Moran Road
Dispositions
Gross Proceeds									34.0
Gain on Sale									$ (15.6)
44874 Moran Road | Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership percentage in the Operating Partnership									75.00%
Dispositions
Gain on Sale									$ (11.7)
1 Solutions Parkway
Dispositions
Gross Proceeds									37.1
Gain on Sale									$ (14.7)
47700 Kato Road and 1055 Page Avenue
Dispositions
Gross Proceeds						$ 37.5
Gain on Sale						$ (1.0)
Data center porfolio
Dispositions
Gross Proceeds				$ 114.5
Gain on Sale				(24.5)
114 Rue Ambroise Croizat
Dispositions
Gross Proceeds		212.0
Gain on Sale		$ (144.3)
Data Center Portfolio, St Louis and Northern Virginia | Disposed of by Sale
Dispositions
Gross Proceeds				$ 114.5
Gain on Sale								$ (24.5)
Number of real estate properties | property				4
Square footage of net rentable space | ft²				454
Data Center Portfolio, St Louis and Northern Virginia | Disposed of by Sale | St. Louis
Dispositions
Number of real estate properties | property				2
Data Center Portfolio, St Louis and Northern Virginia | Disposed of by Sale | Northern Virginia
Dispositions
Number of real estate properties | property				2
Data Center Portfolio, St Louis and Northern Virginia | Held-for-sale
Dispositions
Number of real estate properties | property												4
Subsequent Event | 200 Quannapowitt Parkway | Disposed of by Sale
Dispositions
Gross Proceeds											$ 15.0
Square footage of net rentable space | ft²											211
Subsequent Event | 34551 Ardenwood Boulevard | Disposed of by Sale
Dispositions
Gross Proceeds	$ 73.0
Gain on Sale	$ 25.0
Square footage of net rentable space | ft²	323
Scenario, Forecast | Subsequent Event | 200 Quannapowitt Parkway | Disposed of by Sale
Dispositions
Gain on Sale							$ 0.4