Investments - Principal Investment Income (Loss) from Equity Method Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Investment income (loss)	$ 1,568.4	$ 809.2	$ 2,290.6
Principal investment income (loss) from equity method investments
Schedule of Investments [Line Items]
Investment income (loss)	774.9	189.3	226.4
Principal investment income (loss) from equity method investments | Corporate Private Equity
Schedule of Investments [Line Items]
Investment income (loss)	25.7	22.9	64.8
Principal investment income (loss) from equity method investments | Real Assets
Schedule of Investments [Line Items]
Investment income (loss)	19.5	102.1	151.7
Principal investment income (loss) from equity method investments | Global Credit
Schedule of Investments [Line Items]
Investment income (loss)	718.2	55.8	1.7
Principal investment income (loss) from equity method investments | Investment Solutions
Schedule of Investments [Line Items]
Investment income (loss)	$ 11.5	$ 8.5	$ 8.2