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Fair Value Measurements - Other Investments and Unfunded Commitments (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
NAV of investments at fair value
Fair Value	$ 204.0	$ 211.8
Unfunded Commitments	135.3	127.2
Fair Value Measured at NAV
NAV of investments at fair value
Fair Value	195.5	211.8
Private equity funds
NAV of investments at fair value
Unfunded Commitments	135.3	127.2
Private equity funds | Fair Value Measured at NAV
NAV of investments at fair value
Fair Value	$ 190.2	203.3
Life of funds (generally up to)	15 years
Other funds
NAV of investments at fair value
Unfunded Commitments	$ 0.0	0.0
Other funds | Fair Value Measured at NAV
NAV of investments at fair value
Fair Value	5.3	8.5
Other Investments
NAV of investments at fair value
Fair Value	8.5	0.0
Unfunded Commitments	0.0	0.0
Other Investments | Controlling Interest
NAV of investments at fair value
Fair Value	$ 140.9	$ 137.6

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Fair Value, Investments, Entities That Calculate NAVs, Life of Fund

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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

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