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Investment Securities and Certificates of Deposit - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 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 $ 73,877   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) $ (11) $ (4)  
Equity Securities [Member]      
Schedule Of Marketable Securities [Line Items]      
Investment Securities, Realized and Unrealized Gains (Losses) 2,128 (334)  
Debt Securities Carried by Broker-Dealers [Member]      
Schedule Of Marketable Securities [Line Items]      
Investment Securities, Realized and Unrealized Gains (Losses) (5) (634)  
Debt Securities, Trading 519,987   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) $ 6,228 $ (13,019)