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Fair Value Measurements - Quantitative Information for Level 3 Inputs (Details) $ in Millions | Jun. 30, 2023 USD ($) | Dec. 31, 2022 USD ($)
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Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | $ 247.0 | $ 243.2
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 90.5 | 95.9
Equity Securities, FV-NI | 34.7 | 38.4
Fair Value, Equity Securities FV-NI, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 0.0 | 0.0
Unfunded commitments |  | 192.2
Mezzanine Debt |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 47.0 | 51.6
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 66.2 | 56.0
Senior Debt |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 41.1 | 42.0
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 13.4 | 6.0
Distressed Debt |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 0.0 | 0.0
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 13.3 | 13.0
Secondary Transactions |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 1.7 | 1.7
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 3.1 | 4.2
Hedge Funds |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 0.0 | 0.0
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 0.0 | 0.0
Leveraged Buyout |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 0.7 | 0.6
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 7.8 | 9.0
Equity Funds |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 0.0 | 0.0
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 6.7 | 7.9
Real Estate Funds |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 0.0 | 0.0
Other Funds |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 0.0 | 0.0
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 0.0 | 0.2
Alternative Energy Partnership Investments | Variable Interest Entity, Not Primary Beneficiary |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Outstanding commitments to fund equity method limited liability investments | 0.0 | 0.0
Real Estate |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 0.2 | 0.0
Fair Value, Measurements, Recurring |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Total Assets | 7,240.2 | 7,183.0
Total Liabilities |  | (0.4)
Convertible debt at fair value | 46.2 | 43.3
Derivative Asset | 3.2 | 1.7
Fair Value, Measurements, Recurring | Designated as Hedging Instrument |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative Asset |  | (0.4)
Fair Value, Measurements, Recurring | Limited Liability Companies and Limited Partnerships |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 197.0 | 189.1
Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 11.3 | 10.8
Fair Value, Measurements, Recurring | Other Industries | Common Stock |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 0.8 | 1.2
Significant Unobservable Inputs (Level 3) | Limited Liability Companies and Limited Partnerships |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Unfunded commitments | 201.2 |
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Total Assets | 201.5 | 230.0
Total Liabilities |  | 0.0
Convertible debt at fair value | 0.0 | 0.0
Derivative Asset | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Designated as Hedging Instrument |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative Liability |  | 0.0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Limited Liability Companies and Limited Partnerships |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 1.8 | 1.6
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Other Industries | Common Stock |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 0.4 | 0.5
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Total Assets | 128.3 | 117.0
Total Liabilities |  | 0.0
Convertible debt at fair value | 0.0 |
Derivative Asset | 0.0 | 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Designated as Hedging Instrument |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative Liability |  | 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Limited Liability Companies and Limited Partnerships |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 0.0 | 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 0.0 | 0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Other Industries | Common Stock |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 0.4 | 0.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Total Assets | 6,713.4 | 6,646.9
Derivative Liability |  | 0.4
Total Liabilities |  | 0.4
Convertible debt at fair value | 46.2 | 43.3
Derivative Asset | 3.2 | 1.7
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Limited Liability Companies and Limited Partnerships |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 0.0 | 0.0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 9.5 | 9.2
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Other Industries | Common Stock |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 0.0 | 0.4
Fair Value Measured at Net Asset Value Per Share |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Total Assets | 444.9 | 430.7
Equity Method Investments, Fair Value Disclosure | 225.1 | 217.0
Alternative Investment | 197.0 | 189.1
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments | 110.7 | 96.3
Equity Securities, FV-NI | 6.0 | 8.3
Fair Value Measured at Net Asset Value Per Share | Mezzanine Debt |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Method Investments, Fair Value Disclosure | 125.6 | 114.3
Alternative Investment | 115.8 | 106.0
Fair Value Measured at Net Asset Value Per Share | Senior Debt |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Method Investments, Fair Value Disclosure | 21.8 | 21.6
Alternative Investment | 22.4 | 21.9
Fair Value Measured at Net Asset Value Per Share | Distressed Debt |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Method Investments, Fair Value Disclosure | 8.1 | 9.4
Alternative Investment | 12.5 | 12.5
Fair Value Measured at Net Asset Value Per Share | Secondary Transactions |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Method Investments, Fair Value Disclosure | 8.5 | 9.3
Alternative Investment | 2.8 | 3.5
Fair Value Measured at Net Asset Value Per Share | Hedge Funds |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Method Investments, Fair Value Disclosure | 0.1 | 0.5
Alternative Investment | 14.9 | 18.1
Fair Value Measured at Net Asset Value Per Share | Leveraged Buyout |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Method Investments, Fair Value Disclosure | 9.7 | 8.9
Alternative Investment | 22.5 | 21.6
Fair Value Measured at Net Asset Value Per Share | Equity Funds |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Method Investments, Fair Value Disclosure | 1.2 | 1.2
Alternative Investment | 6.0 | 5.4
Fair Value Measured at Net Asset Value Per Share | Real Estate Funds |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Method Investments, Fair Value Disclosure | 41.5 | 43.3
Fair Value Measured at Net Asset Value Per Share | Other Funds |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Method Investments, Fair Value Disclosure | 8.6 | 8.5
Alternative Investment | 0.0 | 0.1
Fair Value Measured at Net Asset Value Per Share | Alternative Energy Partnership Investments |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Method Investments, Fair Value Disclosure | 16.8 | 16.3
Fair Value Measured at Net Asset Value Per Share | Real Estate |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Alternative Investment | 0.1 | 0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Total Assets | 197.0 | 189.1
Total Liabilities |  | 0.0
Convertible debt at fair value | 0.0 | 0.0
Derivative Asset | 0.0 | 0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Designated as Hedging Instrument |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Derivative Liability |  | 0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Limited Liability Companies and Limited Partnerships |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 197.0 | 189.1
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Other Industries | Preferred Stocks |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | 0.0 | 0.0
Measured at Net Asset Value | Fair Value, Measurements, Recurring | Other Industries | Common Stock |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Equity Securities, FV-NI, Current | $ 0.0 | $ 0.0
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Minimum | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Debt instrument, measurement input | 0.063 | 0.046
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Maximum | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Debt instrument, measurement input | 0.399 | 0.367
Senior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Weighted-average Yield | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Debt instrument, measurement input | 0.153 | 0.109
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Minimum | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Debt instrument, measurement input | 0.063 | 0.088
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Maximum | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Debt instrument, measurement input | 0.225 | 0.225
Junior Debt | Measurement Input, Discount Rate | Non-investment-grade | Market Yield | Weighted-average Yield | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Debt instrument, measurement input | 0.194 | 0.151
Private Placement | Investment-grade | Market Yield | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Total Fair Value | $ 57.4 | $ 56.5
Private Placement | Measurement Input, Discount Rate | Investment-grade | Market Yield | Minimum | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Alternative investment, measurement input | 0.042 | 0.046
Private Placement | Measurement Input, Discount Rate | Investment-grade | Market Yield | Maximum | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Alternative investment, measurement input | 0.143 | 0.145
Private Placement | Measurement Input, Discount Rate | Investment-grade | Market Yield | Weighted-average Yield | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Alternative investment, measurement input | 0.100 | 0.092
Senior Debt | Non-investment-grade | Market Yield | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Total Fair Value | $ 47.8 | $ 72.9
Junior Debt | Non-investment-grade | Market Yield | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Total Fair Value | 33.2 | 42.1
Other | Market Yield | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Total Fair Value | 60.9 | 56.4
Investments in Fixed Maturities | Market Yield | Significant Unobservable Inputs (Level 3) |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Total Fair Value | $ 199.3 | $ 227.9

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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 -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 2: 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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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. 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