Investment in CyrusOne (Details) - USD ($) $ / shares in Units, shares in Millions	3 Months Ended				11 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jan. 24, 2013
Schedule of Equity Method Investments [Line Items]
Proceeds from Equity Method Investment, Dividends or Distributions, Return of Capital						$ 22,200,000	$ 28,400,000	$ 21,300,000
Equity Method Investment Sold, Partnership Units		6.0	14.3	16.0
Sale of Partnership Units, Price Per Unit		$ 28.41	$ 29.88	$ 22.26
Proceeds from Sale of Equity Method Investments	$ 47,600,000	$ 170,300,000	$ 426,000,000	$ 355,900,000		643,900,000	355,900,000	0
Gain on sale of CyrusOne equity method investment	$ 36,300,000	$ 117,700,000	$ 295,200,000	$ 192,800,000		$ 449,200,000	192,800,000	0
Investment owned in Equity Method Investee, Balance, Partnership Units	6.3					6.3			42.6
Cost Method Investment, Ownership Percentage	9.50%					9.50%
Investment owned in Cost Method Investee, Balance, Shares	6.9					6.9
Equity Method Investment Sold, common stock	1.4
Sale of Equity Method Invesment Common Stock, Price Per Share	$ 0
Total ownership interests in CyrusOne, Percentage									69.00%
Investment owned in CyrusOne Inc., Balance, Shares									1.9
Loss from CyrusOne						$ 5,100,000	7,000,000	$ 10,700,000
CyrusOne [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Revenue					$ 248,400,000	399,300,000	330,900,000
Operating income					28,900,000	22,800,000	40,000,000
Net loss					$ (15,600,000)	(20,200,000)	(14,500,000)
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Net investment in real estate	1,392,000,000					1,392,000,000	1,051,400,000
Total assets	2,195,600,000					2,195,600,000	1,571,000,000
Equity Method Investment, Summarized Financial Information, Liabilities [Abstract]
Total liabilities	$ 1,374,000,000					$ 1,374,000,000	$ 854,000,000