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Fair Value Measurements - Fair Value using NAV and Unfunded Commitment by Asset Class (Details) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
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Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Unfunded commitments, equity method limited liability investments | $ 91.3 | $ 85.3
Unfunded commitments, other equity interests at fair value | 130.6 | 110.4
Fair value using NAV, equity securities at modified cost | 22.5 | 32.6
Unfunded commitments, equity securities at modified cost | 0.0 | 0.0
Unfunded commitments, investments in limited liability companies and limited partnerships | 221.9 | 195.7
Mezzanine Debt |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Unfunded commitments, equity method limited liability investments | 40.8 | 43.1
Unfunded commitments, other equity interests at fair value | 67.0 | 67.0
Real Estate |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Unfunded commitments, equity method limited liability investments | 0.0 | 0.0
Senior Debt |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Unfunded commitments, equity method limited liability investments | 48.2 | 39.9
Unfunded commitments, other equity interests at fair value | 8.4 | 10.6
Leveraged Buyout |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Unfunded commitments, equity method limited liability investments | 0.6 | 0.6
Unfunded commitments, other equity interests at fair value | 30.4 | 10.0
Secondary Transactions |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Unfunded commitments, equity method limited liability investments | 1.6 | 1.7
Unfunded commitments, other equity interests at fair value | 1.6 | 3.1
Distressed Debt |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Unfunded commitments, equity method limited liability investments | 0.0 | 0.0
Unfunded commitments, other equity interests at fair value | 15.0 | 13.0
Hedge Fund |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Unfunded commitments, equity method limited liability investments | 0.0 | 0.0
Unfunded commitments, other equity interests at fair value | 0.0 | 0.0
Growth Equity |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Unfunded commitments, equity method limited liability investments | 0.0 | 0.0
Unfunded commitments, other equity interests at fair value | 8.0 | 6.5
Other |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Unfunded commitments, equity method limited liability investments | 0.1 | 0.0
Unfunded commitments, other equity interests at fair value | 0.2 | 0.2
Fair Value Measured at Net Asset Value Per Share |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Fair value using NAV, equity method limited liability investments | 186.3 | 221.7
Fair value using NAV, other equity interests at fair value | 183.6 | 191.4
Fair value using NAV, equity securities at modified cost | 1.8 | 4.8
Total | 371.7 | 417.9
Fair Value Measured at Net Asset Value Per Share | Mezzanine Debt |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Fair value using NAV, equity method limited liability investments | 116.7 | 125.4
Fair value using NAV, other equity interests at fair value | 116.9 | 124.0
Fair Value Measured at Net Asset Value Per Share | Real Estate |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Fair value using NAV, equity method limited liability investments | 27.3 | 41.9
Fair Value Measured at Net Asset Value Per Share | Senior Debt |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Fair value using NAV, equity method limited liability investments | 19.1 | 19.0
Fair value using NAV, other equity interests at fair value | 26.3 | 24.8
Fair Value Measured at Net Asset Value Per Share | Leveraged Buyout |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Fair value using NAV, equity method limited liability investments | 7.5 | 8.6
Fair value using NAV, other equity interests at fair value | 19.2 | 19.0
Fair Value Measured at Net Asset Value Per Share | Secondary Transactions |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Fair value using NAV, equity method limited liability investments | 5.5 | 7.9
Fair value using NAV, other equity interests at fair value | 2.4 | 2.8
Fair Value Measured at Net Asset Value Per Share | Distressed Debt |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Fair value using NAV, equity method limited liability investments | 4.4 | 7.9
Fair value using NAV, other equity interests at fair value | 11.7 | 12.4
Fair Value Measured at Net Asset Value Per Share | Hedge Fund |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Fair value using NAV, equity method limited liability investments | 0.1 | 0.1
Fair value using NAV, other equity interests at fair value | 0.0 | 1.9
Fair Value Measured at Net Asset Value Per Share | Growth Equity |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Fair value using NAV, equity method limited liability investments | 0.0 | 1.2
Fair value using NAV, other equity interests at fair value | 7.0 | 6.4
Fair Value Measured at Net Asset Value Per Share | Other |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Fair value using NAV, equity method limited liability investments | 5.7 | 9.7
Fair value using NAV, other equity interests at fair value | $ 0.1 | $ 0.1

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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/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 107 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-107 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 103 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-103 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 100 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-100 Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 35 -Paragraph 54B -Publisher FASB -URI https://asc.fasb.org/1943274/2147482134/820-10-35-54B Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A + Details
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