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Fair Value Measurements - Investments Measured at Net Asset Value as Practical Expedient (Details) - USD ($) $ in Millions | 9 Months Ended |
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
NAV of investments at fair value |  |  |
Life of funds (in years) |  | 15 years |
Other Investments |  |  |
NAV of investments at fair value |  |  |
Unfunded Commitments | [1] | $ 130.4 | $ 98.8
Other Investments | Fair Value Measured at NAV |  |  |
NAV of investments at fair value |  |  |
Fair Value | [1] | 192.8 | 165.0
Other Investments | Controlling Interest |  |  |
NAV of investments at fair value |  |  |
Fair Value |  | 115.0 | 80.1
Private Equity Funds |  |  |
NAV of investments at fair value |  |  |
Unfunded Commitments | [2] | 130.4 | 98.8
Private Equity Funds | Fair Value Measured at NAV |  |  |
NAV of investments at fair value |  |  |
Fair Value | [2] | 185.4 | 156.1
Other Funds |  |  |
NAV of investments at fair value |  |  |
Unfunded Commitments | [3] | 0.0 | 0.0
Other Funds | Fair Value Measured at NAV |  |  |
NAV of investments at fair value |  |  |
Fair Value | [3] | $ 7.4 | $ 8.9
[1] | Fair value attributable to the controlling interest was $80.1 million and $115.0 million as of December 31, 2017 and September 30, 2018, respectively.
[2] | The Company uses NAV as a practical expedient one quarter in arrears (adjusted for current period calls and distributions) to determine the fair value. These funds primarily invest in a broad range of private equity funds, as well as making direct investments. Distributions will be received as the underlying assets are liquidated over the life of the funds, which is generally up to 15 years
[3] | These are multi-disciplinary funds that invest across various asset classes and strategies, including long/short equity, credit and real estate. Investments are generally redeemable on a daily, monthly or quarterly basis.

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- Definition 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 -Paragraph 6A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 35 -Paragraph 54B -URI http://asc.fasb.org/extlink&oid=99392525&loc=SL7495116-110257 + Details
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