Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Management fee revenue	$ 205.9	$ 226.4	$ 413.4	$ 471.4
U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	40.0	45.5	80.4	93.1
Global / non-U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	116.0	126.2	232.9	254.8
Fixed income
Disaggregation of Revenue [Line Items]
Management fee revenue	6.5	6.8	12.9	13.6
Alternatives
Disaggregation of Revenue [Line Items]
Management fee revenue	$ 43.4	$ 47.9	$ 87.2	$ 109.9