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Restricted Cash (Details) - Tradeweb Markets LLC - USD ($)
Dec. 31, 2018
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Successor        
Entity Information [Line Items]        
Restricted cash $ 1,200,000      
Predecessor        
Entity Information [Line Items]        
Restricted cash   $ 1,200,000 $ 1,200,000 $ 1,000,000