Investments - Income (Loss) from Principal Investments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Schedule of Investments [Line Items]
Income (loss) from principal investments	$ 164.8	$ (181.1)
Global Private Equity
Schedule of Investments [Line Items]
Income (loss) from principal investments	105.6	(50.0)
Global Credit
Schedule of Investments [Line Items]
Income (loss) from principal investments	53.6	(130.6)
Investment Solutions
Schedule of Investments [Line Items]
Income (loss) from principal investments	5.6	(0.5)
Fortitude Group Holdings, LLC
Schedule of Investments [Line Items]
Income (loss) from principal investments	$ 45.5	$ (111.9)