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Fair Value Measurements - Changes in Level 3 Assets and Liabilities (Details) - Significant Unobservable Inputs (Level 3) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Other Investments
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	$ 8.5		$ 0.0
Net realized and unrealized losses	0.0		0.0
Purchases and issuances	0.0		8.5
Settlements and reductions	0.0		0.0
Balance, end of period	8.5		8.5
Net change in unrealized gains (losses) relating to instruments still held at the reporting date	0.0		0.0
Contingent Payment Arrangements
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	0.0	$ 2.0	0.0	$ 1.9
Net realized and unrealized losses	0.0	0.0	0.0	0.1
Purchases and issuances	0.0	0.0	0.0	0.0
Settlements and reductions	0.0	(2.0)	0.0	(2.0)
Balance, end of period	0.0	0.0	0.0	0.0
Net change in unrealized gains (losses) relating to instruments still held at the reporting date	0.0	0.0	0.0	0.0
Affiliate Equity Repurchase Obligations
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	115.1	75.4	19.8	36.2
Net realized and unrealized losses	(2.4)	(0.1)	(3.9)	(0.1)
Purchases and issuances	13.0	18.4	207.0	73.0
Settlements and reductions	(52.4)	(36.3)	(149.6)	(51.7)
Balance, end of period	73.3	57.4	73.3	57.4
Net change in unrealized gains (losses) relating to instruments still held at the reporting date	$ 0.0	$ 0.0	$ 0.0	$ 0.0

X

- Definition

Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Purchases And Issuances

+ References

No definition available.

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- Definition

Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements And Reductions

+ References

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- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings and Other Comprehensive Income (Loss)

+ References

No definition available.

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- Definition

Amount of unrealized gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3) and still held.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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