Fixed Assets - Components of Fixed Assets (Details) - Consolidated Entity Excluding Consolidated Funds - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	$ 154.6	$ 163.7
Accumulated depreciation and amortization	(118.9)	(119.5)
Fixed assets, net	35.7	44.2
Leasehold improvements
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	28.5	30.0
Office equipment
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	8.7	9.5
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	6.3	6.2
Software and web development
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	$ 111.1	$ 118.0