Investments - Income (Loss) from Principal Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule of Investments [Line Items]
Income (loss) from principal investments	$ 75.7	$ 216.7	$ (651.0)	$ 857.2
Corporate Private Equity
Schedule of Investments [Line Items]
Income (loss) from principal investments	19.6	1.5	27.1	13.2
Real Assets
Schedule of Investments [Line Items]
Income (loss) from principal investments	21.1	(64.3)	33.6	29.6
Global Credit
Schedule of Investments [Line Items]
Income (loss) from principal investments	33.9	277.5	(709.9)	803.4
Investment Solutions
Schedule of Investments [Line Items]
Income (loss) from principal investments	1.1	2.0	(1.8)	11.0
Fortitude Group Holdings, LLC
Schedule of Investments [Line Items]
Income (loss) from principal investments	$ 28.7	$ 281.6	$ (703.9)	$ 808.9