Goodwill and Other Intangibles - Other Intangible (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 31, 2016
Other Intangible Assets
Gross Carrying Amount		$ 42,456	$ 42,430
Accumulated Amortization		28,416	27,040
Other intangible assets not subject to amortization		8,900
Amortization expense of other intangible assets		1,276	1,852	$ 3,200
Impairment of intangible assets		325
Impairment of other intangible assets			0
Customer intangibles disposed					$ 6,700
Estimate of future amortization expense for acquired other intangibles
Total		14,040	15,390	$ 24,611
ITG Derivatives
Other Intangible Assets
Impairment of intangible assets	$ 300	300
Trade names
Other Intangible Assets
Gross Carrying Amount		8,529	8,518
Trading rights
Other Intangible Assets
Gross Carrying Amount		339	339
Other
Other Intangible Assets
Gross Carrying Amount		50	50
Estimate of future amortization expense for acquired other intangibles
2018		1,017
2019		865
2020		653
2021		571
2022		571
Thereafter		1,447
Total		5,124
POSIT trade name
Other Intangible Assets
Other intangible assets not subject to amortization		8,400
Customer related intangibles
Other Intangible Assets
Gross Carrying Amount		10,217	10,358
Accumulated Amortization		$ 6,219	5,584
Useful Lives		16 years 10 months 24 days
Proprietary software
Other Intangible Assets
Gross Carrying Amount		$ 23,321	23,165
Accumulated Amortization		$ 22,197	$ 21,456
Useful Lives		8 years 4 months 24 days