Significant Accounting Policies - Basis of Presentation (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Equity method investment, assets	$ 158,356,000	$ 199,452,000
Maximum loss exposure	2,500,000	700,000
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Equity method investment, assets	2,300,000	500,000
Equity Method Investment, Nonconsolidated Investee or Group of Investees | Pzena Global Focused Value Fund
Variable Interest Entity [Line Items]
Equity method investment, assets	2,700,000
Equity Method Investment, Nonconsolidated Investee or Group of Investees | VIEs that are not consolidated
Variable Interest Entity [Line Items]
Equity method investment, assets	$ 200,000,000.0	$ 247,800,000