Fair Value Measurements - Fair Value using NAV and Unfunded Commitment by Asset Class (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded commitments, equity method limited liability investments	$ 95.9	$ 198.5
Unfunded commitments, other equity interests at fair value	96.3	102.3
Fair value using NAV, equity securities at modified cost	38.4	32.3
Unfunded commitments, equity securities at modified cost	0.0	0.0
Unfunded commitments, investments in limited liability companies and limited partnerships	192.2	300.8
Mezzanine Debt
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded commitments, equity method limited liability investments	51.6	43.3
Unfunded commitments, other equity interests at fair value	56.0	53.7
Senior Debt
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded commitments, equity method limited liability investments	42.0	46.7
Unfunded commitments, other equity interests at fair value	6.0	15.1
Distressed Debt
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded commitments, equity method limited liability investments	0.0	100.1
Unfunded commitments, other equity interests at fair value	13.0	20.0
Secondary Transactions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded commitments, equity method limited liability investments	1.7	8.3
Unfunded commitments, other equity interests at fair value	4.2	6.8
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	0.1
Unfunded commitments, other equity interests at fair value	9.0	6.0
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	7.9	0.7
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
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	0.2	0.0
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	217.0	241.9
Fair value using NAV, other equity interests at fair value	189.1	325.0
Fair value using NAV, equity securities at modified cost	8.3	7.7
Total	414.4	574.6
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	114.3	120.0
Fair value using NAV, other equity interests at fair value	106.0	129.3
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	21.6	27.5
Fair value using NAV, other equity interests at fair value	21.9	29.9
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	9.4	21.7
Fair value using NAV, other equity interests at fair value	12.5	44.9
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	9.3	11.7
Fair value using NAV, other equity interests at fair value	3.5	4.0
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	0.5	8.7
Fair value using NAV, other equity interests at fair value	18.1	82.7
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	8.9	8.7
Fair value using NAV, other equity interests at fair value	21.6	32.2
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	0.7
Fair value using NAV, other equity interests at fair value	5.4	2.0
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	43.3	29.9
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	8.5	13.0
Fair value using NAV, other equity interests at fair value	$ 0.1	$ 0.0