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Goodwill and Intangible Assets - Goodwill (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Changes in goodwill    
Balance as of December 31 $ 1,148,926 $ 1,148,926
Operating Segments | Market Making    
Changes in goodwill    
Balance as of December 31 755,292 755,292
Operating Segments | Execution Services    
Changes in goodwill    
Balance as of December 31 393,634 393,634
Corporate    
Changes in goodwill    
Balance as of December 31 $ 0 $ 0