Fixed Assets and Lease Commitments (Details) - Consolidated Entity Excluding Consolidated Funds - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	$ 147.6	$ 130.7
Accumulated depreciation and amortization	(81.8)	(81.7)
Fixed assets, net	65.8	49.0
Depreciation and amortization	17.2	14.5	$ 11.7
Leasehold improvements
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	37.2	38.0
Office equipment
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	20.0	30.1
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	9.0	8.9
Building
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	2.9	2.9
Software and web development
Property, Plant and Equipment [Line Items]
Fixed assets, at cost	$ 78.5	$ 50.8