Significant Accounting Policies - Basis of Presentation (Details) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Variable Interest Entity [Line Items]
Equity method investment, assets	$ 202,958,000	$ 188,687,000
Maximum loss exposure	4,400,000	3,600,000
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Equity method investment, assets	4,200,000	3,200,000
Equity Method Investment, Nonconsolidated Investee or Group of Investees | Pzena Global Focused Value Fund
Variable Interest Entity [Line Items]
Equity method investment, assets	12,800,000
Equity Method Investment, Nonconsolidated Investee or Group of Investees | VIEs that are not consolidated
Variable Interest Entity [Line Items]
Equity method investment, assets	$ 252,100,000	$ 265,900,000