Fixed Assets - Components of Fixed Assets (Details) - Consolidated Entity Excluding Consolidated Funds - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	$ 148.2	$ 154.9
Accumulated depreciation and amortization	(98.0)	(92.1)
Fixed assets, net	50.2	62.8
Leasehold improvements
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	30.4	31.6
Office equipment
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	8.4	14.6
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	6.2	10.9
Software and web development
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	$ 103.2	$ 97.8