Investments in Real Estate (Dispositions) (Details) - Disposed of by Sale - USD ($) $ in Millions									12 Months Ended
	Nov. 04, 2019	Sep. 21, 2018	May 31, 2018	May 22, 2018	Apr. 19, 2018	Mar. 14, 2018	Feb. 09, 2018	Jan. 25, 2018	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross Proceeds									$ 291.7
Gain (loss) on sale									$ 80.4
Mapletree Portfolio [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross Proceeds	$ 996.6
Gain (loss) on sale	$ 266.0
200 Quannapowitt Parkway
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross Proceeds								$ 15.0
Gain (loss) on sale								$ (0.4)
34551 Ardenwood Boulevard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross Proceeds							$ 73.3
Gain (loss) on sale							$ 25.3
3065 Gold Camp Drive
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross Proceeds						$ 14.2
Gain (loss) on sale						5.4
11085 Sun Center Drive
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross Proceeds						36.8
Gain (loss) on sale						$ 9.1
Austin Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross Proceeds					$ 47.6
Gain (loss) on sale					$ 12.0
2010 East Centennial Circle
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross Proceeds				$ 5.5
Gain (loss) on sale				$ (0.5)
1125 Energy Park Drive
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross Proceeds			$ 7.0
Gain (loss) on sale			$ 2.8
360 Spear Street
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross Proceeds		$ 92.3
Gain (loss) on sale		$ 26.7