Fair Value Measurements - Balance Sheet Grouping (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Using NAV	$ 22.1	$ 32.6
Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unfunded Commitments	77.0	85.3
Unfunded Commitments	0.0	0.0
Unfunded Commitments	184.8	195.7
Mezzanine Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unfunded Commitments	38.6	43.1
Unfunded Commitments	63.4	67.0
Real Estate Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unfunded Commitments	0.0	0.0
Senior Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unfunded Commitments	36.1	39.9
Unfunded Commitments	6.5	10.6
Leveraged Buyout \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unfunded Commitments	0.6	0.6
Unfunded Commitments	12.9	10.0
Secondary Transactions \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unfunded Commitments	1.6	1.7
Unfunded Commitments	1.6	3.1
Distressed Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unfunded Commitments	0.0	0.0
Unfunded Commitments	15.0	13.0
Equity Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unfunded Commitments	0.0	0.0
Unfunded Commitments	8.2	6.5
Hedge Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unfunded Commitments	0.0	0.0
Unfunded Commitments	0.0	0.0
Other Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unfunded Commitments	0.1	0.0
Unfunded Commitments	0.2	0.2
Measured at Net Asset Value \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Using NAV	202.6	221.7
Fair Value Using NAV	189.2	191.4
Unfunded Commitments	107.8	110.4
Fair Value Using NAV	1.8	4.8
Fair Value Using NAV	393.6	417.9
Measured at Net Asset Value \| Mezzanine Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Using NAV	127.9	125.4
Fair Value Using NAV	122.6	124.0
Measured at Net Asset Value \| Real Estate Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Using NAV	27.1	41.9
Measured at Net Asset Value \| Senior Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Using NAV	20.2	19.0
Fair Value Using NAV	26.0	24.8
Measured at Net Asset Value \| Leveraged Buyout \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Using NAV	7.9	8.6
Fair Value Using NAV	20.2	19.0
Measured at Net Asset Value \| Secondary Transactions \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Using NAV	6.0	7.9
Fair Value Using NAV	2.5	2.8
Measured at Net Asset Value \| Distressed Debt \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Using NAV	5.0	7.9
Fair Value Using NAV	10.9	12.4
Measured at Net Asset Value \| Equity Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Using NAV	0.5	1.2
Fair Value Using NAV	6.9	6.4
Measured at Net Asset Value \| Hedge Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Using NAV	0.1	0.1
Fair Value Using NAV	0.0	1.9
Measured at Net Asset Value \| Other Funds \| Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Using NAV	7.9	9.7
Fair Value Using NAV	$ 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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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