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Investment Securities and Certificates of Deposit - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Schedule Of Marketable Securities [Line Items]          
Other Payables to Broker-Dealers and Clearing Organizations         $ 99,983
Certificates of Deposit, at Carrying Value $ 114,512   $ 114,512   0
Certificates of Deposit          
Schedule Of Marketable Securities [Line Items]          
Cash and Cash Equivalents Maturity     4 months    
Debt Securities [Member]          
Schedule Of Marketable Securities [Line Items]          
Investment Securities, Realized Gains (Losses)   $ 2 $ (11) $ 75  
Equity Securities [Member]          
Schedule Of Marketable Securities [Line Items]          
Investment Securities, Realized and Unrealized Gains (Losses) (520) 92 1,340 (154)  
Debt Securities Carried by Broker-Dealers [Member]          
Schedule Of Marketable Securities [Line Items]          
Investment Securities, Realized and Unrealized Gains (Losses) 26   17 (1,240)  
Debt Securities, Trading 564,877   564,877   550,026
Debt Securities Carried by Broker-Dealers [Member] | US Treasury Bill Securities [Member]          
Schedule Of Marketable Securities [Line Items]          
Debt Securities, Trading         $ 99,983
Investment Funds [Member]          
Schedule Of Marketable Securities [Line Items]          
Investment Securities, Realized and Unrealized Gains (Losses) $ (195) $ 7,798 $ 15,807 $ 5,008