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Fair Value Measurements - Quantitative Information used in Valuing Level 3 Liabilities (Details) - Discounted Cash Flow - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Contingent Payment Arrangements
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value liabilities	$ 4.2	$ 9.4
Growth rate (as a percent)	7.00%
Discount rate (as a percent)	15.00%
Contingent Payment Arrangements | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)		7.00%
Discount rate (as a percent)		15.00%
Contingent Payment Arrangements | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)		8.00%
Discount rate (as a percent)		16.00%
Affiliate Equity Obligations
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value liabilities	$ 36.9	$ 49.2
Affiliate Equity Obligations | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)	0.00%	0.00%
Discount rate (as a percent)	12.00%	12.00%
Affiliate Equity Obligations | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)	11.00%	11.00%
Discount rate (as a percent)	16.00%	16.00%

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- Definition

Represents percentage of assumed growth rates, used as an input to measure fair value.

+ References

No definition available.

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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