Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Management fee revenue	$ 170.7	$ 205.9	$ 349.2	$ 413.4
Quant & Solutions | Global / non-U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	82.2	93.6	167.4	184.1
Alternatives | Alternatives
Disaggregation of Revenue [Line Items]
Management fee revenue	41.8	43.5	83.2	87.3
Liquid Alpha | Global / non-U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	19.9	22.3	38.7	48.7
Liquid Alpha | Fixed income
Disaggregation of Revenue [Line Items]
Management fee revenue	6.1	6.5	12.4	12.9
Liquid Alpha | U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	$ 20.7	$ 40.0	$ 47.5	$ 80.4