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Significant Accounting Policies - Breakdown of Revenues Between Fixed and Variable (Details) - USD ($) $ in Thousands	3 Months Ended					9 Months Ended		12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue Recognition
Financial Designation, Predecessor and Successor [Fixed List]	Successor	Successor				Successor	Predecessor	Successor	Successor	Predecessor
Gross revenues		$ 178,637					$ 505,771	$ 775,566		$ 562,968
Compensation expense		9,400					15,900	50,700		26,100
Transaction fees
Revenue Recognition
Gross revenues		97,130					273,751	423,583		267,020
Commissions
Revenue Recognition
Gross revenues		32,840					79,830	149,365		96,745
Other
Revenue Recognition
Gross revenues		$ 2,148					$ 8,209	$ 8,252		$ 4,669
Variable
Revenue Recognition
Financial Designation, Predecessor and Successor [Fixed List]		Successor					Predecessor	Successor
Gross revenues		$ 96,833					$ 258,761	$ 437,743
Variable \| Transaction fees
Revenue Recognition
Gross revenues		73,800					208,049	325,178
Variable \| Subscription Fees including Refinitiv market data
Revenue Recognition
Gross revenues		425					1,305	1,736
Variable \| Commissions
Revenue Recognition
Gross revenues		$ 22,608					49,367	109,995
Variable \| Other
Revenue Recognition
Gross revenues							$ 40	$ 834
Fixed
Revenue Recognition
Financial Designation, Predecessor and Successor [Fixed List]		Successor					Predecessor	Successor
Gross revenues		$ 81,804					$ 247,010	$ 337,823
Fixed \| Transaction fees
Revenue Recognition
Gross revenues		23,330					65,702	98,405
Fixed \| Subscription Fees including Refinitiv market data
Revenue Recognition
Gross revenues		46,094					142,676	192,630
Fixed \| Commissions
Revenue Recognition
Gross revenues		10,232					30,463	39,370
Fixed \| Other
Revenue Recognition
Gross revenues		$ 2,148					$ 8,169	$ 7,418
Tradeweb Markets LLC
Revenue Recognition
Financial Designation, Predecessor and Successor [Fixed List]	Successor	Successor	Predecessor	Predecessor	Predecessor
Gross revenues	$ 186,792	$ 178,637	$ 165,253	$ 171,015	$ 169,503
Tradeweb Markets LLC \| Transaction fees
Revenue Recognition
Gross revenues	102,640	97,130	92,582	91,030	90,139
Tradeweb Markets LLC \| Commissions
Revenue Recognition
Gross revenues	34,197	32,840	24,394	27,553	27,883
Tradeweb Markets LLC \| Other
Revenue Recognition
Gross revenues	$ 1,894	$ 2,148	$ 2,587	$ 2,704	$ 2,918