Summary of Significant Accounting Policies - Investments in Unconsolidated Variable Interest Entities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Variable Interest Entity [Line Items]
Investments	$ 9,955.3	$ 10,767.9
Total assets	21,176.0	21,403.0	$ 21,250.4
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Investments	1,118.4	1,124.0
Accrued performance allocations	492.3	406.0
Management fee receivables	65.1	49.6
Total assets	$ 1,675.8	$ 1,579.6