Income Taxes - Components of Provision (Tradeweb Markets LLC and Subsidiaries) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current:
Financial Designation, Predecessor and Successor [Fixed List]	Successor	Successor	Successor	Predecessor	Successor	Successor	Predecessor
Federal					$ 21,373
State and Local		$ 1,235		$ 5,739	11,537		$ 4,331
Foreign		1,212		3,559	4,368		2,748
Total current tax expense		2,447		9,298	37,278		7,079
Deferred:
Federal		680		1,085	(88)		(433)
State and local		288		1,517	18,194		(517)
Foreign					(3,082)
Total deferred tax expense		968		2,602	15,024		(950)
Total provision for income taxes		$ 3,415		$ 11,900	$ 52,302		$ 6,129