Goodwill and Other Intangibles - Other Intangible (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	May 27, 2016	Dec. 31, 2015
Other Intangible Assets
Gross Carrying Amount	$ 42,619		$ 42,619			$ 55,814
Accumulated Amortization	26,747		26,747			31,203
Other intangible assets not subject to amortization	8,900		8,900
Amortization expense of other intangible assets	300	$ 800	1,508	$ 2,400
Net of accumulated amortization	15,872		15,872			24,611
Impairment of other intangible assets			0
Trade name
Other Intangible Assets
Gross Carrying Amount	8,523		8,523			8,545
Trading rights
Other Intangible Assets
Gross Carrying Amount	339		339			339
Other
Other Intangible Assets
Gross Carrying Amount	50		50			50
POSIT trade name
Other Intangible Assets
Other intangible assets not subject to amortization	8,400		8,400
Customer related intangibles
Other Intangible Assets
Gross Carrying Amount	10,458		10,458			23,322
Accumulated Amortization	5,446		$ 5,446			10,400
Useful Lives			14 years 3 months 18 days
Proprietary software
Other Intangible Assets
Gross Carrying Amount	23,249		$ 23,249			23,558
Accumulated Amortization	$ 21,301		$ 21,301			$ 20,803
Useful Lives			6 years 3 months 18 days
Sale of subsidiaries | ITG Investment Research
Other Intangible Assets
Net of accumulated amortization					$ 6,700