Segment Reporting - Financial Information (Details) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment reporting
Number of operating segments | segment				4
Total revenues	$ 120,769			$ 380,730
(Loss) income before income tax (benefit) expense	2,759		$ (1,955)	8,402	$ 6,943
Identifiable assets	891,759			891,759		$ 784,856
Expense incurred as potential settlement with SEC				12,000
Legal expenses	200			800
Restructuring charges	3,436			10,601
Litigation settlement amount				18,000
US | Restatement Adjustment
Segment reporting
Restructuring charges	3,400			10,600
Increase (decrease) in expenses			(2,700)		(8,000)
Charge incurred to increase liability	900			900
Canada | Restatement Adjustment
Segment reporting
Increase (decrease) in expenses			700		2,000
Europe | Restatement Adjustment
Segment reporting
Increase (decrease) in expenses			1,300		3,800
Asia Pacific | Restatement Adjustment
Segment reporting
Increase (decrease) in expenses			700		2,200
Corporate
Segment reporting
Total revenues	638			1,532
U.S. Operations
Segment reporting
Total revenues	45,398			141,910
(Loss) income before income tax (benefit) expense	(2,736)			(5,090)
Identifiable assets	389,240			389,240
Canadian Operations
Segment reporting
Total revenues	16,190			51,967
(Loss) income before income tax (benefit) expense	1,833			8,712
Identifiable assets	96,951			96,951
European Operations
Segment reporting
Total revenues	40,019			127,760
(Loss) income before income tax (benefit) expense	9,377			31,726
Identifiable assets	315,351			315,351
Asia Pacific Operations
Segment reporting
Total revenues	18,524			57,561
(Loss) income before income tax (benefit) expense	3,852			12,532
Identifiable assets	90,217			90,217
Corporate Activity
Segment reporting
Total revenues	638			1,532
(Loss) income before income tax (benefit) expense	$ (9,567)			$ (39,478)
Matrix Holding Group
Segment reporting
Legal expenses		$ 800
ITG Derivatives
Segment reporting
Impairment of intangible assets					300
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Segment reporting
Total revenues			114,531		356,947
(Loss) income before income tax (benefit) expense			(1,955)		6,943
Identifiable assets			794,045		794,045
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Corporate
Segment reporting
Total revenues			490		1,170
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | U.S. Operations
Segment reporting
Total revenues			47,403		153,559
(Loss) income before income tax (benefit) expense			(6,333)		(12,646)
Identifiable assets			338,000		338,000
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Canadian Operations
Segment reporting
Total revenues			15,055		47,521
(Loss) income before income tax (benefit) expense			738		6,034
Identifiable assets			75,332		75,332
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | European Operations
Segment reporting
Total revenues			36,256		111,707
(Loss) income before income tax (benefit) expense			7,662		27,170
Identifiable assets			323,142		323,142
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Asia Pacific Operations
Segment reporting
Total revenues			15,327		42,990
(Loss) income before income tax (benefit) expense			2,204		4,117
Identifiable assets			57,571		57,571
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Corporate Activity
Segment reporting
Total revenues			490		1,170
(Loss) income before income tax (benefit) expense			$ (6,226)		$ (17,732)