Investment in CyrusOne (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
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	$ 170.3	$ 426.0		$ 355.9	$ 596.3	$ 355.9
Equity Method Investment, Realized Gain (Loss) on Disposal	$ 117.7	$ 295.2	$ 0.0	$ 192.8	$ 412.9	192.8
Total ownership interests in CyrusOne, Percentage	11.00%				11.00%
Investment owned in CyrusOne Inc., Balance, Shares	1.9				1.9
Investment owned in CyrusOne LP, Balance, Partnership Units	6.3				6.3
(Income) loss from CyrusOne equity method investment	$ 0.8		0.0		$ 5.2	1.9
Equity Method Investments, Fair Value Disclosure	269.0				269.0		$ 785.0
Dividends received from CyrusOne	4.5		6.0		19.5	22.4
Operating income (loss)	$ 36.2		47.8		$ 103.0	145.5
CyrusOne [Member]
Schedule of Equity Method Investments [Line Items]
Dividends Payable, Amount Per Share	$ 0.315				$ 0.315
Revenue	$ 111.2		84.8		$ 286.0	244.0
Operating income (loss)	7.5		9.6		11.7	28.8
Net income	(5.3)		0.2		(19.0)	(2.7)
CyrusOne [Member]
Schedule of Equity Method Investments [Line Items]
Revenue from Related Parties	$ 0.3		$ 0.4		$ 1.0	$ 1.3