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Investment Securities and Certificates of Deposit - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 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 $ 122,510   $ 122,510   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)   $ 77 $ (11) $ 73  
Equity Securities [Member]          
Schedule Of Marketable Securities [Line Items]          
Investment Securities, Realized and Unrealized Gains (Losses) (267) 88 1,860 (246)  
Debt Securities Carried by Broker-Dealers [Member]          
Schedule Of Marketable Securities [Line Items]          
Investment Securities, Realized and Unrealized Gains (Losses) (4) (1,111) (9) (1,240)  
Debt Securities, Trading 519,888   519,888   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) $ 9,774 $ 10,284 $ 16,002 $ (2,790)