Summary of Significant Accounting Policies - Investments in Unconsolidated Variable Interest Entities (Details) - Unconsolidated VIEs - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Investments	$ 957.4	$ 1,029.5
Accrued performance revenues	132.2	160.2
Management fee receivables	49.3	35.4
Total	$ 1,138.9	$ 1,225.1