Fair Value Measurements - Fair Value using NAV and Unfunded Commitment by Asset Class (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded commitments, equity method limited liability investments	$ 85.3	$ 95.9
Unfunded commitments, other equity interests at fair value	110.4	96.3
Fair value using NAV, equity securities at modified cost	32.6	38.4
Unfunded commitments, equity securities at modified cost	0.0	0.0
Unfunded commitments, investments in limited liability companies and limited partnerships	195.7	192.2
Mezzanine Debt
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded commitments, equity method limited liability investments	43.1	51.6
Unfunded commitments, other equity interests at fair value	67.0	56.0
Senior Debt
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded commitments, equity method limited liability investments	39.9	42.0
Unfunded commitments, other equity interests at fair value	10.6	6.0
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	13.0	13.0
Secondary Transactions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded commitments, equity method limited liability investments	1.7	1.7
Unfunded commitments, other equity interests at fair value	3.1	4.2
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
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	10.0	9.0
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	6.5	7.9
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
Unfunded commitments, other equity interests at fair value	0.2	0.0
Other
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	0.2
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	221.7	217.0
Fair value using NAV, other equity interests at fair value	191.4	189.1
Fair value using NAV, equity securities at modified cost	4.8	8.3
Total	417.9	414.4
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	125.4	114.3
Fair value using NAV, other equity interests at fair value	124.0	106.0
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	19.0	21.6
Fair value using NAV, other equity interests at fair value	24.8	21.9
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	7.9	9.4
Fair value using NAV, other equity interests at fair value	12.4	12.5
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	7.9	9.3
Fair value using NAV, other equity interests at fair value	2.8	3.5
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	0.5
Fair value using NAV, other equity interests at fair value	1.9	18.1
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	8.6	8.9
Fair value using NAV, other equity interests at fair value	19.0	21.6
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	1.2	1.2
Fair value using NAV, other equity interests at fair value	6.4	5.4
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	41.9	43.3
Fair value using NAV, other equity interests at fair value	0.1	0.0
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	9.7	8.5
Fair value using NAV, other equity interests at fair value	$ 0.0	$ 0.1