Fair Value Measurements and the Fair Value Option - Schedule of Changes in Level 3 Investments (Details) - Equity Method Investments [Member] - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 406.9	$ 409.7	$ 380.7	$ 356.7
Unrealized and realized gains	10.9	5.7	39.7	31.6
Unrealized and realized losses	(5.3)	(2.7)	(7.7)	(1.0)
Contributions	101.3	5.0	115.9	25.2
Distributions	(35.5)	(55.4)	(50.2)	(74.5)
Other	0.0	(0.4)	(0.1)	23.9
Ending balance	$ 478.3	$ 361.9	$ 478.3	$ 361.9

Fair Value, Measurements with Unobservable Inputs Reconciliation Recurring Basis, Asset, Other

No definition available.

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- Definition

Fair Value, Measurements with Unobservable Inputs Reconciliation Recurring Basis, Asset, Unrealized and Realized Gains

+ References

No definition available.

+ Details

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- Definition

Fair Value, Measurements with Unobservable Inputs Reconciliation Recurring Basis, Asset, Unrealized and Realized Losses

+ References

No definition available.

+ Details

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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

Amount of purchases of financial instrument classified as an asset 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)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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- Definition

Amount of sale of financial instrument classified as an asset 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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- Definition

Fair value of financial instrument classified as an asset 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=99377789&loc=d3e19207-110258

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