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Income Taxes - Reconciliation of the Statutory Rate (Details) - Tradeweb Markets LLC	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of the statutory tax rate
U.S. federal tax at statutory rate			21.00%
Successor
Reconciliation of the statutory tax rate
U.S. federal tax at statutory rate	21.00%
State and local taxes - net of federal benefit	4.70%
Foreign taxes	3.70%
Tax Cuts and Jobs Act provisional tax charge	0.00%
LLC flow-through structure	(19.00%)
Effective tax rate	10.40%
Predecessor
Reconciliation of the statutory tax rate
U.S. federal tax at statutory rate		21.00%		35.00%	35.00%
State and local taxes - net of federal benefit		5.10%		2.80%	1.60%
Foreign taxes		2.50%		3.10%	3.10%
Tax Cuts and Jobs Act provisional tax charge		0.00%		2.20%	0.00%
LLC flow-through structure		(20.20%)		(36.30%)	(40.50%)
Effective tax rate		8.40%		6.80%	(0.80%)

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- Definition

Percentage of reported income tax expense (benefit) in excess of (less than) expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operations, attributable to increase (decrease) in income tax rate and transition tax on accumulated earnings of controlled foreign corporation deemed repatriated pursuant to Tax Cuts and Jobs Act of 2017.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to equity in earnings (loss) of unconsolidated subsidiaries exempt from income taxes.

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