Investments - Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net Investment Income [Line Items]
Net consolidated Funds’ investment gains	$ 6.0	$ 7.0	$ 17.3	$ 25.2
Consolidated Entity Excluding Consolidated Funds
Net Investment Income [Line Items]
Investment income (loss)	3.9	2.3	(3.2)	11.4
Consolidated Entity Excluding Consolidated Funds | Realized and unrealized gains (losses) on other investments held at fair value
Net Investment Income [Line Items]
Investment income (loss)	2.9	1.5	(5.4)	9.3
Consolidated Entity Excluding Consolidated Funds | Earnings from equity-accounted investments in Affiliates
Net Investment Income [Line Items]
Investment income (loss)	1.0	0.8	2.2	2.1
Consolidated Funds
Net Investment Income [Line Items]
Net consolidated Funds’ investment gains	2.1	4.7	20.5	13.8
Consolidated Funds | Realized and unrealized gains (losses) on other investments held at fair value
Net Investment Income [Line Items]
Net consolidated Funds’ investment gains	2.9	(0.4)	(7.6)	2.6
Consolidated Funds | Earnings from equity-accounted investments in Affiliates
Net Investment Income [Line Items]
Net consolidated Funds’ investment gains	$ (0.8)	$ 5.1	$ 28.1	$ 11.2