Segment Reporting - Reconciliation of Fund Level Fee Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Total Reportable Segments
Segment Reporting Information [Line Items]
Fund management fees	$ 394.2	$ 395.7	$ 1,187.2	$ 1,216.7
Adjustments
Segment Reporting Information [Line Items]
Adjustments	(30.4)	(36.2)	(95.7)	(112.9)
Fund management fees
Segment Reporting Information [Line Items]
Fund management fees	363.8	359.5	1,091.5	1,103.8
Fund management fees | Total Reportable Segments
Segment Reporting Information [Line Items]
Fund management fees	389.1	384.6	1,157.0	1,180.7
Fund management fees | Adjustments
Segment Reporting Information [Line Items]
Fund management fees	$ (6.2)	$ (5.3)	$ (17.0)	$ (16.1)