Supplementary Financial Information - Expenses as a Percent (unaudited) (Details) - USD ($) $ in Thousands		3 Months Ended											6 Months Ended		12 Months Ended
	Apr. 19, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total revenues (as a percent)		100.00%	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%
Expenses (as a percent)
Acquisition-related costs		$ 8,000
Compensation and employee benefits (as a percent)		37.10%	37.50%	35.10%	34.80%	35.70%	40.80%	37.80%	38.60%
Transaction processing (as a percent)		20.10%	19.30%	20.20%	20.60%	21.30%	20.50%	21.00%	20.60%
Occupancy and equipment (as a percent)		11.70%	13.20%	11.70%	11.20%	18.40%	13.00%	12.10%	12.90%
Telecommunications and data processing services (as a percent)		10.30%	10.50%	10.10%	9.60%	9.60%	10.60%	10.00%	10.00%
Restructuring charges (as a percent)		0.00%	2.80%		5.40%
Other general and administrative (as a percent)		19.00%	14.10%	22.70%	13.50%	15.00%	16.30%	14.60%	14.30%
Interest expense (as a percent)		0.40%	0.40%	0.40%	0.40%	0.40%	0.40%	0.40%	0.40%
Total expenses (as a percent)		98.70%	97.70%	100.20%	95.50%	100.40%	101.60%	95.90%	96.80%
Income (loss) before income tax expense		1.30%	2.30%	(0.20%)	4.50%	(0.40%)	(1.60%)	4.10%	3.20%
Income tax expense (as a percent)		2.00%	2.10%	2.20%	1.20%	1.60%	39.30%	0.40%	(1.20%)
Net income (loss) (as a percent)		(0.70%)	0.20%	(2.40%)	3.30%	(2.00%)	(40.90%)	3.70%	4.40%
Write off of fixed assets						$ 8,100									$ 400
Corporate income tax rate as a percentage						35.00%									21.00%	35.00%	35.00%
Valuation allowance		$ 58,879				$ 58,448									$ 58,879	$ 58,448
Net deferred tax assets		2,865				3,152		$ 42,300							2,865	3,152
Increase (decrease) in valuation allowance							$ 48,100								(400)
Legal and other settlement related costs		8,000		$ 1,200
Impairment of intangible assets																$ 325
Restructuring charges		1	$ 3,436		$ 7,165										$ 10,601		$ 9,620
Litigation settlement amount	$ 18,000
ITG Derivatives
Expenses (as a percent)
Increase (decrease) in accrual for the consolidation of office space		500	900
Impairment of intangible assets							300
Settlement agreement for termination of Former CEO
Expenses (as a percent)
Legal and other settlement related costs														$ 2,700
Net settlement charge														4,800
Insurance recovery												$ 500		$ 500
Proceeds from insurer to settle claims											$ 900
Settlement Agreement With SEC Involving Pre-released ADRs
Expenses (as a percent)
Legal and other settlement related costs													$ 2,900
Settlement with the SEC, related to an investigation into the operational features of U.S. POSIT and access to U.S. POSIT data
Expenses (as a percent)
Legal and other settlement related costs		$ 100	900	200
Litigation settlement amount				$ 12,000
Employee separation and related costs
Expenses (as a percent)
Increase (decrease) in accrual for the consolidation of office space			3,400
Restructuring charges					$ 7,200					$ 5,300
Employee separation and related costs | Settlement with the SEC, related to an investigation into the operational features of U.S. POSIT and access to U.S. POSIT data
Expenses (as a percent)
Restructuring charges			$ 3,400
Reduction in trading and sales organizations and closing matched-book securities operations
Expenses (as a percent)
Restructuring charges												$ 4,300
U.S.
Expenses (as a percent)
Valuation allowance							42,300
Increase (decrease) in valuation allowance						$ (900)
Matrix Holding Group
Expenses (as a percent)
Legal and other settlement related costs							$ 800