Fair Value Measurements - Changes in Level 3 Assets and Liabilities (Details) - Significant Unobservable Inputs (Level 3) - Affiliate equity repurchase obligations - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	$ 73.3	$ 57.4	$ 19.8	$ 36.2
Net realized and unrealized losses	(0.2)	0.7	(4.1)	0.6
Purchases and issuances	85.8	12.1	292.8	85.1
Settlements and reductions	(89.0)	(33.4)	(238.6)	(85.1)
Balance, end of period	69.9	36.8	69.9	36.8
Net change in unrealized (gains) losses relating to instruments still held at the reporting date	$ 0.0	$ 0.0	$ 0.0	$ 0.0

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

No definition available.

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- Definition

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

+ References

No definition available.

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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.

+ Details

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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.

+ References

No definition available.

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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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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