Segment Reporting - Financial Information (Details)			3 Months Ended											6 Months Ended		12 Months Ended
	Apr. 19, 2018 USD ($)	May 27, 2016 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment reporting
Number of operating segments | segment																4
Total revenues			$ 128,746,000	$ 120,769,000	$ 128,477,000	$ 131,484,000	$ 126,747,000	$ 114,531,000	$ 121,581,000	$ 120,835,000						$ 509,476,000	$ 483,694,000	$ 469,052,000
(Loss) income before income tax expense (benefit)			1,697,000	2,759,000	(254,000)	5,897,000	(418,000)	(1,955,000)	$ 5,087,000	$ 3,811,000						10,099,000	6,525,000	(43,240,000)
Identifiable assets			1,021,075,000				784,856,000				$ 775,285,000			$ 775,285,000		1,021,075,000	784,856,000	775,285,000
Capital purchases																(13,801,000)	(19,402,000)	(21,315,000)
Depreciation, Amortization and Accretion, Net																43,860,000	45,153,000	43,523,000
Non-cash share-based compensation																29,762,000	20,238,000	25,620,000
Expense incurred as potential settlement with SEC					12,000,000
Legal expenses			8,000,000		$ 1,200,000
Restructuring charges			1,000	$ 3,436,000		7,165,000										10,601,000		9,620,000
Write off of fixed assets							8,100,000									400,000
Litigation settlement amount	$ 18,000,000
Pre-tax gain on sale																608,000
Impairment of intangible assets																	325,000
Settlement agreement for termination of Former CEO
Segment reporting
Legal expenses															$ 2,700,000
Proceeds from insurer to settle claims												$ 900,000
Net litigation settlement amount															(4,800,000)
Insurance recovery													$ 500,000		$ 500,000
Settlement Agreement With SEC Involving Pre-released ADRs
Segment reporting
Legal expenses														2,900,000
Loss of Profits Due to Outage of Outsourced Primary Data Centre
Segment reporting
Proceeds from insurer to settle claims													2,400,000
Employee separation and related costs
Segment reporting
Restructuring charges						$ 7,200,000					5,300,000
Reduction in trading and sales organizations and closing matched-book securities operations
Segment reporting
Restructuring charges													$ 4,300,000
Corporate
Segment reporting
Total revenues																3,004,000	1,720,000	3,711,000
(Loss) income before income tax expense (benefit)																(49,672,000)	(32,393,000)	(70,694,000)
Depreciation, Amortization and Accretion, Net																1,082,000	1,338,000
Non-cash share-based compensation																7,232,000	2,079,000	5,722,000
U.S. Operations | Restatement Adjustment
Segment reporting
(Loss) income before income tax expense (benefit)																	10,700,000	12,500,000
U.S. Operations | Operating Segments
Segment reporting
Total revenues																191,391,000	204,990,000	229,655,000
(Loss) income before income tax expense (benefit)																(7,885,000)	(13,830,000)	1,319,000
Identifiable assets			478,254,000				342,487,000				396,419,000			396,419,000		478,254,000	342,487,000	396,419,000
Capital purchases																(6,701,000)	(13,278,000)	(15,547,000)
Depreciation, Amortization and Accretion, Net																31,818,000	33,307,000	32,896,000
Non-cash share-based compensation																11,446,000	9,543,000	10,642,000
Canadian Operations | Restatement Adjustment
Segment reporting
(Loss) income before income tax expense (benefit)																	2,700,000	3,100,000
Canadian Operations | Operating Segments
Segment reporting
Total revenues																71,973,000	63,294,000	61,822,000
(Loss) income before income tax expense (benefit)																13,224,000	7,942,000	8,474,000
Identifiable assets			132,815,000				108,457,000				80,943,000			80,943,000		132,815,000	108,457,000	80,943,000
Capital purchases																(1,413,000)	(1,183,000)	(2,549,000)
Depreciation, Amortization and Accretion, Net																2,187,000	2,420,000	2,402,000
Non-cash share-based compensation																3,108,000	2,390,000	2,383,000
European Operations | Restatement Adjustment
Segment reporting
(Loss) income before income tax expense (benefit)																	5,100,000	6,300,000
European Operations | Operating Segments
Segment reporting
Total revenues																168,643,000	151,472,000	125,945,000
(Loss) income before income tax expense (benefit)																40,251,000	36,061,000	19,198,000
Identifiable assets			325,660,000				274,836,000				236,071,000			236,071,000		325,660,000	274,836,000	236,071,000
Capital purchases																(3,332,000)	(3,289,000)	(1,886,000)
Depreciation, Amortization and Accretion, Net																6,416,000	6,094,000	6,741,000
Non-cash share-based compensation																6,468,000	5,267,000	5,948,000
Asia Pacific Operations | Restatement Adjustment
Segment reporting
(Loss) income before income tax expense (benefit)																	2,900,000	3,100,000
Asia Pacific Operations | Operating Segments
Segment reporting
Total revenues																74,465,000	62,218,000	47,919,000
(Loss) income before income tax expense (benefit)																14,181,000	8,745,000	(1,537,000)
Identifiable assets			$ 84,346,000				$ 59,076,000				$ 61,852,000			$ 61,852,000		84,346,000	59,076,000	61,852,000
Capital purchases																(2,355,000)	(1,652,000)	(1,333,000)
Depreciation, Amortization and Accretion, Net																2,357,000	1,994,000	1,484,000
Non-cash share-based compensation																$ 1,508,000	$ 959,000	$ 925,000
Sale of subsidiaries | ITG Investment Research
Segment reporting
Cash proceeds from sale		$ 12,000,000
Pre-tax gain on sale		$ 21,000
Matrix Holding Group
Segment reporting
Legal expenses								$ 800,000