Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Management fee revenue	$ 196.4	$ 229.6	$ 609.8	$ 701.0
Quant & Solutions | Global / non-U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	92.7	95.9	276.8	288.7
Alternatives | Alternatives
Disaggregation of Revenue [Line Items]
Management fee revenue	37.2	54.6	124.5	164.5
Liquid Alpha | Global / non-U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	21.9	27.1	70.6	89.1
Liquid Alpha | Fixed income
Disaggregation of Revenue [Line Items]
Management fee revenue	6.6	6.7	19.5	20.3
Liquid Alpha | U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	$ 38.0	$ 45.3	$ 118.4	$ 138.4