Summary of Significant Accounting Polices and Adoption of New Accounting Pronouncements (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Other assets	$ (246.8)			$ (246.8)		$ (263.1)
Unconsolidated investments	789.7			789.7		519.3
Income from unconsolidated investments	24.0	$ 17.6		60.9	$ 57.4
Accounting Standards Update 2014-09 [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Income from unconsolidated investments	$ 7.6			$ 24.7
Accounting Standards Update 2014-09 [Member] | Restatement Adjustment [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Other assets						32.9
Unconsolidated investments						$ 32.9
Income from unconsolidated investments		4.7	$ 4.7		8.6
Investment management, property services and research fees		$ 4.7			$ 8.6