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Investments - Equity Method Investments and Investments Carried Under Measurement Alternative (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Schedule Of Equity Method Investments [Line Items]    
Equity method investments $ 37,659 $ 40,000
Investments carried under measurement alternative 349 349
Total equity method and investments carried under measurement alternative $ 38,008 40,349
Advanced Markets Holdings [Member]    
Schedule Of Equity Method Investments [Line Items]    
Investment ownership percentage amount 43.00%  
Equity method investments $ 8,867 10,259
Freedom International Brokerage [Member]    
Schedule Of Equity Method Investments [Line Items]    
Investment ownership percentage amount 45.00%  
Equity method investments $ 9,800 10,142
China Credit BGC Money Broking Company Limited [Member]    
Schedule Of Equity Method Investments [Line Items]    
Investment ownership percentage amount 33.00%  
Equity method investments $ 15,119 12,214
Other Investees [Member]    
Schedule Of Equity Method Investments [Line Items]    
Equity method investments $ 3,873 $ 7,385