Investments - Components of Investment Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Investment income (loss)	$ 1,568.4	$ 809.2	$ 2,290.6
Performance allocations - realized
Investment [Line Items]
Investment income (loss)	354.8	693.8	1,062.8
Performance allocations - unrealized
Investment [Line Items]
Investment income (loss)	444.3	(70.9)	995.8
Performance allocations
Investment [Line Items]
Investment income (loss)	799.1	622.9	2,058.6
Principal investment income (loss) from equity method investments - realized
Investment [Line Items]
Investment income (loss)	189.5	122.9	68.1
Principal investment income (loss) from equity method investments - unrealized
Investment [Line Items]
Investment income (loss)	585.4	66.4	158.3
Principal investment income (loss) from equity method investments
Investment [Line Items]
Investment income (loss)	774.9	189.3	226.4
Principal investment income (loss) from investments in CLOs and other investments - realized
Investment [Line Items]
Investment income (loss)	1.0	1.5	2.3
Principal investment income (loss) from investments in CLOs and other investments- unrealized
Investment [Line Items]
Investment income (loss)	(6.6)	(4.5)	3.3
Principal investment income (loss) from investments in CLOs and other investments
Investment [Line Items]
Investment income (loss)	$ (5.6)	$ (3.0)	$ 5.6