Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Management fee revenue	$ 178.2	$ 196.4	$ 527.4	$ 609.8
Quant & Solutions | Global / non-U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	87.9	92.7	255.3	276.8
Alternatives | Alternatives
Disaggregation of Revenue [Line Items]
Management fee revenue	43.2	37.2	126.4	124.5
Liquid Alpha | Global / non-U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	17.8	21.9	56.5	70.6
Liquid Alpha | Fixed income
Disaggregation of Revenue [Line Items]
Management fee revenue	6.1	6.6	18.5	19.5
Liquid Alpha | U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	$ 23.2	$ 38.0	$ 70.7	$ 118.4