Fair Value Measurements - Balance Sheet Grouping (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity Securities, FV-NI	$ 24.8	$ 32.6
Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	80.8	85.3
Fair Value, Equity Securities FV-NI, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Unfunded commitments	189.6	195.7
Mezzanine Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	42.1	43.1
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	65.9	67.0
Real Estate Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.0	0.0
Senior Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	36.4	39.9
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	7.7	10.6
Leveraged Buyout \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.6	0.6
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	9.2	10.0
Secondary Transactions \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	1.6	1.7
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	3.0	3.1
Distressed Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [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	13.7	13.0
Equity Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [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	9.1	6.5
Hedge Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [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
Other Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.1	0.0
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	0.2	0.2
Fair Value Measured at Net Asset Value Per Share \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity Method Investments, Fair Value Disclosure	205.1	221.7
Alternative Investment	189.3	191.4
Fair Value, Alternative Investments, Entities That Calculate Net Asset Value Per Share, Unfunded Commitments	108.8	110.4
Equity Securities, FV-NI	4.6	4.8
Investments, Fair Value Disclosure	399.0	417.9
Fair Value Measured at Net Asset Value Per Share \| Mezzanine Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity Method Investments, Fair Value Disclosure	126.0	125.4
Alternative Investment	124.1	124.0
Fair Value Measured at Net Asset Value Per Share \| Real Estate Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity Method Investments, Fair Value Disclosure	27.2	41.9
Fair Value Measured at Net Asset Value Per Share \| Senior Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity Method Investments, Fair Value Disclosure	22.4	19.0
Alternative Investment	25.1	24.8
Fair Value Measured at Net Asset Value Per Share \| Leveraged Buyout \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity Method Investments, Fair Value Disclosure	7.9	8.6
Alternative Investment	19.2	19.0
Fair Value Measured at Net Asset Value Per Share \| Secondary Transactions \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity Method Investments, Fair Value Disclosure	6.8	7.9
Alternative Investment	2.8	2.8
Fair Value Measured at Net Asset Value Per Share \| Distressed Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity Method Investments, Fair Value Disclosure	4.9	7.9
Alternative Investment	11.4	12.4
Fair Value Measured at Net Asset Value Per Share \| Equity Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity Method Investments, Fair Value Disclosure	0.8	1.2
Alternative Investment	6.6	6.4
Fair Value Measured at Net Asset Value Per Share \| Hedge Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity Method Investments, Fair Value Disclosure	0.1	0.1
Alternative Investment	0.0	1.9
Fair Value Measured at Net Asset Value Per Share \| Other Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity Method Investments, Fair Value Disclosure	9.0	9.7
Alternative Investment	$ 0.1	$ 0.1

Fair Value, Alternative Investments, Entities That Calculate Net Asset Value, Unfunded Commitments

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Fair Value, Equity Method Investments, Entities That Calculate Net Asset Value, Unfunded Commitments

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Fair Value, Equity Securities FV-NI, Entities That Calculate Net Asset Value, Unfunded Commitments

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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.

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Fair value portion of investments accounted under the equity method.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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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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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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