Supplementary Financial Information - Expenses as a Percent (unaudited) (Details) - USD ($) $ in Thousands		3 Months Ended								6 Months Ended		12 Months Ended
	Aug. 12, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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)
Compensation and employee benefits (as a percent)		35.70%	40.80%	37.80%	38.60%	35.90%	43.30%	40.10%	42.10%
Transaction processing (as a percent)		21.30%	20.50%	21.00%	20.60%	20.50%	20.00%	18.30%	18.30%
Occupancy and equipment (as a percent)		18.40%	13.00%	12.10%	12.90%	12.00%	13.30%	11.70%	11.20%
Telecommunications and data processing services (as a percent)		9.60%	10.60%	10.00%	10.00%	11.10%	13.20%	12.30%	11.80%
Restructuring charges (as a percent)						4.40%		3.60%
Other general and administrative (as a percent)		15.00%	16.30%	14.60%	14.30%	17.40%	36.40%	21.60%	19.00%
Interest expense (as a percent)		0.40%	0.40%	0.40%	0.40%	0.50%	0.50%	0.50%	0.40%
Total expenses (as a percent)		100.40%	101.60%	95.90%	96.80%	101.80%	126.70%	108.10%	102.80%
Income (loss) before income tax expense		(0.40%)	(1.60%)	4.10%	3.20%	(1.80%)	(26.70%)	(8.10%)	(2.80%)
Income tax expense (as a percent)		1.60%	39.30%	0.40%	(1.20%)	(6.60%)	(3.70%)	(3.70%)	(0.90%)
Net income (loss) (as a percent)		(2.00%)	(40.90%)	3.70%	4.40%	4.80%	(23.00%)	(4.40%)	(1.90%)
Write off of fixed assets		$ 8,100
Corporate income tax rate as a percentage		35.00%											35.00%	35.00%	35.00%
Valuation allowance		$ 58,448				$ 20,979				$ 20,979			$ 58,448	$ 20,979
Net deferred tax assets		3,152		$ 42,300		38,688				38,688			3,152	38,688
Increase (decrease) in valuation allowance			$ 48,100
Legal and other settlement related costs															$ 4,900
Impairment of intangible assets													325
Restructuring charges						5,265		$ 4,355						9,620
Decrease in income tax reserve due to resolution of uncertain tax positions						7,300				7,300				$ 7,300
Matrix Holding Group
Expenses (as a percent)
Legal and other settlement related costs			800										800
ITG Derivatives
Expenses (as a percent)
Impairment of intangible assets			$ 300										300
Settlement agreement for termination of Former CEO
Expenses (as a percent)
Legal and other settlement related costs								2,700	$ 2,700		$ 2,700
Net settlement charge								4,800	4,800		4,800
Insurance recovery								500	$ 500		$ 500
Proceeds from insurer to settle claims							$ 900
Settlement Agreement With SEC Involving Prereleased ADRs
Expenses (as a percent)
Legal and other settlement related costs						1,300	1,600			$ 2,900
Litigation settlement amount						2,400	$ 22,100
Settlement with S E C for Investigation into Proprietary Trading Pilot
Expenses (as a percent)
Litigation settlement amount	$ (20,300)
Employee separation and related costs
Expenses (as a percent)
Restructuring charges						$ 5,300
Reduction in trading and sales organizations and closing matched-book securities operations
Expenses (as a percent)
Restructuring charges								$ 4,300
Forecast
Expenses (as a percent)
Corporate income tax rate as a percentage												21.00%
U.S.
Expenses (as a percent)
Valuation allowance		37,700		$ 42,300									$ 37,700
Increase (decrease) in valuation allowance		$ 900