Fixed Assets - Components of Fixed Assets (Details) - Consolidated Entity Excluding Consolidated Funds - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	$ 154.2	$ 148.2
Accumulated depreciation and amortization	(106.5)	(98.0)
Fixed assets, net	47.7	50.2
Leasehold improvements
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	29.0	30.4
Office equipment
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	8.3	8.4
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	6.1	6.2
Software and web development
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	$ 110.8	$ 103.2