Revenues from Contracts with Customers (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 498,544,000	$ 184,339,000	$ 116,503,000
Total revenue	1,530,082,000	1,878,718,000	1,027,982,000
Receivables related to revenue from contracts with customers	53,600,000	1,700,000
Impairment losses on receivables	0
Deferred revenue related to contracts with customers	8,600,000
Revenue recognized	32,600,000
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenue	1,463,895,000	1,873,398,000
Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenue	66,187,000	5,320,000
Commissions, net
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	380,927,000	179,019,000
Technology services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		5,320,000
Workflow technology
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	82,610,000
Analytics
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	35,007,000
Other sources of revenue
Disaggregation of Revenue [Line Items]
Other sources of revenue	1,031,538,000	1,694,379,000
Operating Segments | Market Making
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	23,526,000	28,813,000
Total revenue	1,032,072,000	1,384,475,000	836,707,000
Operating Segments | Market Making | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenue	1,032,072,000	1,384,475,000
Operating Segments | Market Making | Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenue	0	0
Operating Segments | Market Making | Commissions, net
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	23,526,000	28,813,000
Operating Segments | Market Making | Technology services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0
Operating Segments | Market Making | Workflow technology
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Operating Segments | Market Making | Analytics
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Operating Segments | Market Making | Other sources of revenue
Disaggregation of Revenue [Line Items]
Other sources of revenue	1,008,546,000	1,355,662,000
Operating Segments | Execution Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	475,018,000	155,526,000
Total revenue	493,908,000	496,333,000	99,135,000
Operating Segments | Execution Services | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenue	427,721,000	491,013,000
Operating Segments | Execution Services | Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenue	66,187,000	5,320,000
Operating Segments | Execution Services | Commissions, net
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	357,401,000	150,206,000
Operating Segments | Execution Services | Technology services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		5,320,000
Operating Segments | Execution Services | Workflow technology
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	82,610,000
Operating Segments | Execution Services | Analytics
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	35,007,000
Operating Segments | Execution Services | Other sources of revenue
Disaggregation of Revenue [Line Items]
Other sources of revenue	18,890,000	340,807,000
Corporate
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Total revenue	4,102,000	(2,090,000)	$ 92,140,000
Corporate | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenue	4,102,000	(2,090,000)
Corporate | Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenue	0	0
Corporate | Commissions, net
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Corporate | Technology services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0
Corporate | Workflow technology
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Corporate | Analytics
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Corporate | Other sources of revenue
Disaggregation of Revenue [Line Items]
Other sources of revenue	$ 4,102,000	$ (2,090,000)