Geographic Information and Business Segments - Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total revenue	$ 1,530,082	$ 1,878,718	$ 1,027,982
Income (loss) before income taxes and noncontrolling interest	(115,982)	696,363	113,164
Operating Segments | Market Making
Segment Reporting Information [Line Items]
Total revenue	1,032,072	1,384,475	836,707
Income (loss) before income taxes and noncontrolling interest	109,189	422,648	74,633
Operating Segments | Execution Services
Segment Reporting Information [Line Items]
Total revenue	493,908	496,333	99,135
Income (loss) before income taxes and noncontrolling interest	(126,930)	325,043	(12,519)
Corporate
Segment Reporting Information [Line Items]
Total revenue	4,102	(2,090)	92,140
Income (loss) before income taxes and noncontrolling interest	$ (98,241)	$ (51,328)	$ 51,050