Revenues from Contracts with Customers (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 139,627,000	$ 40,252,000	$ 359,894,000	$ 140,661,000
Total revenues	385,364,000	295,123,000	1,126,812,000	1,438,303,000
Revenue related to performance obligations that had been satisfied in previous periods	300,000		1,000,000.0
Receivables related to revenue from contracts with customers	54,000,000.0		54,000,000.0		$ 1,700,000
Impairment losses on receivables			0
Deferred revenue related to contracts with customers	10,000,000.0		10,000,000.0
Revenue recognized	8,100,000		23,100,000
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	365,859,000	294,790,000	1,080,259,000	1,433,314,000
Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	19,505,000	333,000	46,553,000	4,989,000
Commissions, net
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	105,617,000	39,919,000	277,059,000	135,672,000
Technology services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		333,000		4,989,000
Workflow technology
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	24,041,000		58,369,000
Analytics
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,969,000		24,466,000
Other sources of revenue
Disaggregation of Revenue [Line Items]
Other sources of revenue	245,737,000	254,871,000	766,918,000	1,297,642,000
Operating Segments | Market Making
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,252,000	6,587,000	17,213,000	21,886,000
Total revenues	250,797,000	264,370,000	765,022,000	984,601,000
Operating Segments | Market Making | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	250,797,000	264,370,000	765,022,000	984,601,000
Operating Segments | Market Making | Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Operating Segments | Market Making | Commissions, net
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,252,000	6,587,000	17,213,000	21,886,000
Operating Segments | Market Making | Technology services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Operating Segments | Market Making | Workflow technology
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Operating Segments | Market Making | Analytics
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Operating Segments | Market Making | Other sources of revenue
Disaggregation of Revenue [Line Items]
Other sources of revenue	243,545,000	257,783,000	747,809,000	962,715,000
Operating Segments | Execution Services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	132,375,000	33,665,000	342,681,000	118,775,000
Total revenues	134,369,000	34,206,000	359,911,000	458,759,000
Operating Segments | Execution Services | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	114,864,000	33,873,000	313,358,000	453,770,000
Operating Segments | Execution Services | Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	19,505,000	333,000	46,553,000	4,989,000
Operating Segments | Execution Services | Commissions, net
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	98,365,000	33,332,000	259,846,000	113,786,000
Operating Segments | Execution Services | Technology services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		333,000		4,989,000
Operating Segments | Execution Services | Workflow technology
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	24,041,000		58,369,000
Operating Segments | Execution Services | Analytics
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,969,000		24,466,000
Operating Segments | Execution Services | Other sources of revenue
Disaggregation of Revenue [Line Items]
Other sources of revenue	1,994,000	541,000	17,230,000	339,984,000
Corporate
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Total revenues	198,000	(3,453,000)	1,879,000	(5,057,000)
Corporate | Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total revenues	198,000	(3,453,000)	1,879,000	(5,057,000)
Corporate | Services transferred over time
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Corporate | Commissions, net
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Corporate | Technology services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers		0		0
Corporate | Workflow technology
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Corporate | Analytics
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Corporate | Other sources of revenue
Disaggregation of Revenue [Line Items]
Other sources of revenue	$ 198,000	$ (3,453,000)	$ 1,879,000	$ (5,057,000)