Intangible Assets (Schedule of finite-lived intangible assets by major class) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Gross Book Value	$ 1,659.7	$ 1,481.1
Accumulated Amortization	(101.0)	(81.7)
Net Book Value	1,558.7	1,399.4
Management contracts
Gross Book Value	68.8	67.7
Accumulated Amortization	(66.3)	(56.6)
Net Book Value	2.5	11.1
Customer relationships [Member]
Gross Book Value	40.0	40.0
Accumulated Amortization	(25.3)	(21.9)
Net Book Value	14.7	18.1
Other [Member]
Gross Book Value	27.6	9.8
Accumulated Amortization	(9.4)	(3.2)
Net Book Value	18.2	6.6
Finite-lived Intangible Assets Acquired	$ 16.9	$ 9.8
Weighted Average Amortization Period (years)	1 year 11 months	1 year 1 month
Management contracts
Gross Book Value	$ 1,523.3	$ 1,363.6
Net Book Value	$ 1,523.3	$ 1,363.6

Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

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- Definition

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

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- Definition

Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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- Definition

Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition.

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