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Affiliate Equity - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Affiliate Equity
Distributions paid to affiliate partners	$ 352.2	$ 354.1	$ 431.4
Payments to acquire interest in affiliates	174.7	115.8	130.8
Issuance of interest in affiliates	9.0	11.8	$ 6.1
Other Assets
Affiliate Equity
Due from affiliates	12.4	22.9
Other Liabilities
Affiliate Equity
Due to affiliates	$ 49.2	$ 12.1
Minimum
Affiliate Equity
Affiliate equity, conditional right to put interest, period	5 years
Maximum
Affiliate Equity
Affiliate equity, conditional right to put interest, period	15 years

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Affiliate Equity, Conditional Right to Put Interest, Period

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[Line Items] for Affiliate Equity [Table]

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For an unclassified balance sheet, amount of receivables due from an entity that is affiliated with the reporting entity by means of direct or indirect ownership.

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Amount of payable due to an entity that is affiliated with the reporting entity by means of direct or indirect ownership.

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Amount of cash outflow in the form of ordinary dividends provided by the non-wholly owned subsidiary to noncontrolling interests.

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The cash outflow associated with the acquisition of a controlling interest in another entity or an entity that is related to it but not strictly controlled (for example, an unconsolidated subsidiary, affiliate, joint venture or equity method investment).

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The cash inflow associated with the amount received from the sale of a business segment or subsidiary or sale of an entity that is related to it but not strictly controlled during the period (for example, an unconsolidated subsidiary, affiliate, joint venture or equity method investment).

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