Segment Reporting - Financial Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment reporting
Number of operating segments | segment	4
Total Revenues	$ 131,484
Income (loss) before income tax expense (benefit)	5,897	$ 3,811
Identifiable assets	800,668		$ 784,856
Restructuring charges	7,165
Litigation settlement amount	18,000
Reduction in trading and sales organizations and closing matched-book securities operations
Segment reporting
Restructuring charges	7,200
US | Restatement Adjustment
Segment reporting
Increase (decrease) in expenses		(2,700)
Canada | Restatement Adjustment
Segment reporting
Increase (decrease) in expenses		700
Europe | Restatement Adjustment
Segment reporting
Increase (decrease) in expenses		1,300
Asia Pacific | Restatement Adjustment
Segment reporting
Increase (decrease) in expenses		700
U.S. Operations
Segment reporting
Total Revenues	48,486
Income (loss) before income tax expense (benefit)	(1,366)
Identifiable assets	338,020
Canadian Operations
Segment reporting
Total Revenues	18,047
Income (loss) before income tax expense (benefit)	3,829
Identifiable assets	78,553
European Operations
Segment reporting
Total Revenues	44,830
Income (loss) before income tax expense (benefit)	11,909
Identifiable assets	321,069
Asia Pacific Operations
Segment reporting
Total Revenues	19,607
Income (loss) before income tax expense (benefit)	4,680
Identifiable assets	63,026
Corporate Activity
Segment reporting
Total Revenues	514
Income (loss) before income tax expense (benefit)	(13,155)
Corporate
Segment reporting
Total Revenues	$ 514
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Segment reporting
Total Revenues		120,835
Income (loss) before income tax expense (benefit)		3,811
Identifiable assets		884,656
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | U.S. Operations
Segment reporting
Total Revenues		53,393
Income (loss) before income tax expense (benefit)		(3,943)
Identifiable assets		412,323
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Canadian Operations
Segment reporting
Total Revenues		16,482
Income (loss) before income tax expense (benefit)		2,831
Identifiable assets		90,276
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | European Operations
Segment reporting
Total Revenues		36,712
Income (loss) before income tax expense (benefit)		9,938
Identifiable assets		330,309
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Asia Pacific Operations
Segment reporting
Total Revenues		13,943
Income (loss) before income tax expense (benefit)		1,114
Identifiable assets		51,748
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Corporate Activity
Segment reporting
Total Revenues		305
Income (loss) before income tax expense (benefit)		(6,129)
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Corporate
Segment reporting
Total Revenues		$ 305