Investment in CyrusOne (Details) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($) shares	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($) shares	Jun. 30, 2014 shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	Jan. 24, 2013 shares
Schedule of Equity Method Investments [Line Items]
Equity Method Investment Sold, Partnership Units | shares							14.3	16.0		20.3	16.0
Marketable Securities, Realized Gain (Loss)	$ 83.0	$ 5.1	$ 33.3	$ 118.6	$ 36.3	$ 117.7	$ 295.2		$ 157.0	$ 449.2	$ 192.8
Sale of Stock, Number of Shares Issued in Transaction | shares	2.0								4.1	1.4
Proceeds from Sale of Available-for-sale Securities, Equity	$ 100.0								$ 189.7	$ 643.9	355.9
Investment owned in Equity Method Investee, Balance, Partnership Units | shares					6.3					6.3		42.6
Available for sale securities, ownership percentage					9.50%					9.50%
Dividend Income, Operating									6.4
Proceeds from Dividends Received									$ 2.1	$ 22.2	28.4
Total ownership interests in CyrusOne, Percentage												69.00%
Investment owned in CyrusOne Inc., Balance, Shares | shares												1.9
Loss from CyrusOne										$ 5.1	$ 7.0
Investment owned in Available for Sale Securities, Balance, Shares | shares		2.8			6.9				2.8	6.9
Available-for-sale Securities		$ 128.0							$ 128.0