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Fair Value Measurements - Changes in Level 3 Financial Assets and Liabilities (Details) - Affiliate Equity Repurchase Obligations - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Changes in level 3 assets and liabilities
Balance, beginning of period	$ 19.8	$ 36.2
Net realized and unrealized (gains) losses	(4.3)	0.1
Purchases and issuances	310.6	118.6
Settlements and reductions	(304.1)	(135.1)
Balance, end of period	22.0	19.8
Net change in unrealized (gains) losses relating to instruments still held at the reporting date	$ 0.0	$ 0.0

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- 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

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.

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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

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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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