Geographic Information and Business Segments - Segments (Details) $ in Thousands		3 Months Ended	12 Months Ended
	Jul. 19, 2017 business segment	Sep. 30, 2017 segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of businesses managed and operated | business	1
Number of reportable segments | segment	1
Number of operating segments | segment	1	2
Number of non-operating segments | segment		1
Total revenue			$ 1,878,718	$ 1,027,982	$ 702,272
Income (loss) before income taxes and noncontrolling interest			696,363	113,164	179,591
Market Making
Segment Reporting Information [Line Items]
Total revenue			1,384,475	836,707	691,884
Income (loss) before income taxes and noncontrolling interest			422,648	74,633	176,145
Execution Services
Segment Reporting Information [Line Items]
Total revenue			496,333	99,135	10,352
Income (loss) before income taxes and noncontrolling interest			325,043	(12,519)	4,403
Corporate
Segment Reporting Information [Line Items]
Total revenue			(2,090)	92,140	36
Income (loss) before income taxes and noncontrolling interest			$ (51,328)	$ 51,050	$ (957)