Fair Value Measurements - Fair Value using NAV and Unfunded Commitment by Asset Class (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
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