Segment Reporting - Financial Information (Details)				1 Months Ended	3 Months Ended								6 Months Ended		12 Months Ended
	May 27, 2017 USD ($)	May 27, 2016 USD ($)	Dec. 22, 2015 USD ($)	Dec. 31, 2015 USD ($)	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 ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)
Segment reporting
Number of operating segments | segment															4
Total Revenues					$ 126,747,000	$ 114,531,000	$ 121,581,000	$ 120,835,000	$ 119,589,000	$ 104,185,000	$ 120,610,000	$ 124,668,000			$ 483,694,000	$ 469,052,000	$ 634,803,000
(Loss) income before income tax (benefit) expense					(418,000)	(1,955,000)	5,087,000	3,811,000	(2,146,000)	(27,798,000)	(9,658,000)	(3,638,000)			6,525,000	(43,240,000)	121,226,000
Identifiable assets				$ 1,709,022,000	784,856,000				775,285,000				$ 775,285,000		784,856,000	775,285,000	1,709,022,000
Capital purchases															19,402,000	21,315,000	11,905,000
Depreciation and amortization															45,153,000	43,523,000	44,151,000
Non-cash share-based compensation															20,238,000	25,620,000	16,667,000
Write off of fixed assets					8,100,000
Impairment of intangible assets															325,000
Tax charge				6,500,000	2,000,000	45,012,000	$ 444,000	$ (1,491,000)	(7,862,000)	(3,887,000)	(4,441,000)	(1,132,000)			45,965,000	(17,322,000)	29,656,000
Restructuring charges									5,265,000		4,355,000					9,620,000
Legal Fees																	4,900,000
Pre-tax gain on sale																21,000	107,699,000
Aggregate amount payable																		$ 20,300,000
Settlement agreement for termination of Former CEO
Segment reporting
Proceeds from insurer to settle claims										900,000
Legal Fees											2,700,000	2,700,000		$ 2,700,000
Net litigation settlement amount											(4,800,000)	(4,800,000)		(4,800,000)
Insurance recovery											500,000	$ 500,000		$ 500,000
Settlement Agreement With SEC Involving Prereleased ADRs
Segment reporting
Litigation settlement amount									2,400,000	22,100,000
Legal Fees									1,300,000	$ 1,600,000			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									5,300,000
Reduction in trading and sales organizations and closing matched-book securities operations
Segment reporting
Restructuring charges											$ 4,300,000
Operating Segments | U.S. Operations
Segment reporting
Total Revenues															204,990,000	229,655,000	285,230,000
(Loss) income before income tax (benefit) expense															(24,473,000)	(11,133,000)	16,000,000
Identifiable assets				1,304,995,000	342,487,000				396,419,000				396,419,000		342,487,000	396,419,000	1,304,995,000
Capital purchases															13,278,000	15,547,000	7,748,000
Depreciation and amortization															33,307,000	32,896,000	33,971,000
Non-cash share-based compensation															9,543,000	10,642,000	10,725,000
Operating Segments | Canadian Operations
Segment reporting
Total Revenues															63,294,000	61,822,000	63,028,000
(Loss) income before income tax (benefit) expense															10,608,000	11,564,000	10,643,000
Identifiable assets				76,561,000	108,457,000				80,943,000				80,943,000		108,457,000	80,943,000	76,561,000
Capital purchases															1,183,000	2,549,000	2,175,000
Depreciation and amortization															2,420,000	2,402,000	2,247,000
Non-cash share-based compensation															2,390,000	2,383,000	1,466,000
Operating Segments | European Operations
Segment reporting
Total Revenues															151,472,000	125,945,000	129,729,000
(Loss) income before income tax (benefit) expense															41,103,000	25,470,000	28,244,000
Identifiable assets				262,095,000	274,836,000				236,071,000				236,071,000		274,836,000	236,071,000	262,095,000
Capital purchases															3,289,000	1,886,000	1,334,000
Depreciation and amortization															6,094,000	6,741,000	6,560,000
Non-cash share-based compensation															5,267,000	5,948,000	4,505,000
Operating Segments | Asia Pacific Operations
Segment reporting
Total Revenues															62,218,000	47,919,000	48,179,000
(Loss) income before income tax (benefit) expense															11,680,000	1,553,000	1,620,000
Identifiable assets				65,371,000	$ 59,076,000				$ 61,852,000				$ 61,852,000		59,076,000	61,852,000	65,371,000
Capital purchases															1,652,000	1,333,000	648,000
Depreciation and amortization															1,994,000	1,484,000	1,373,000
Non-cash share-based compensation															959,000	925,000	866,000
Corporate
Segment reporting
Total Revenues															1,720,000	3,711,000	108,637,000
(Loss) income before income tax (benefit) expense															(32,393,000)	(70,694,000)	64,719,000
Depreciation and amortization															1,338,000
Non-cash share-based compensation															2,079,000	$ 5,722,000	$ (895,000)
Matrix Holding Group
Segment reporting
Legal Fees						800,000									800,000
ITG Derivatives
Segment reporting
Total Revenues															5,300,000
Impairment of intangible assets						$ 300,000									$ 300,000
Sale of subsidiaries | Energy research business
Segment reporting
Cash proceeds from sale			$ 120,500,000	120,500,000
Pre-tax gain on sale			$ 107,699,000	$ 107,700,000
Sale of subsidiaries | ITG Investment Research
Segment reporting
Cash proceeds from sale	$ 12,000,000	$ 12,000,000
Pre-tax gain on sale	$ 21,000	$ 21,000