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Investments - Summary of Cost and Fair Value of Investments in Proprietary Funds (Details) - Proprietary Funds - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]    
Cost $ 687 $ 758
Gross Unrealized Gains 202 164
Gross Unrealized (Losses) (3)  
Fair Value $ 886 $ 922