Divestitures and Acquisitions - Divestitures (Details) - USD ($)				1 Months Ended	3 Months Ended								12 Months Ended
	May 27, 2017	May 27, 2016	Dec. 22, 2015	Dec. 31, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Divestitures
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
Impairment of intangible assets													$ 325,000
Ownership percentage for equity method investments					20.00%								20.00%
Legal and related costs															4,900,000
Pre-tax gain on sale														$ 21,000	$ 107,699,000
ITG Investment Research | Sale of subsidiaries
Divestitures
Cash proceeds from sale	$ 12,000,000	$ 12,000,000
Carrying value of net assets disposed		(7,502,000)
Direct selling costs		(4,477,000)
After-tax gain on sale of business													$ 50,000
Pre-tax gain on sale	$ 21,000	$ 21,000
Energy research business | Sale of subsidiaries
Divestitures
Number of years as exclusive sales partner			2 years
Currency translation losses			$ 200,000
Cash proceeds from sale			120,500,000	$ 120,500,000
Working capital adjustment			(1,615,000)
Carrying value of net assets disposed			(3,025,000)
Direct selling costs			(8,161,000)
After-tax gain on sale of business			91,400,000
Pre-tax gain on sale			$ 107,699,000	$ 107,700,000
Matrix Holding Group
Divestitures
Legal and related costs						800,000							800,000
ITG Derivatives
Divestitures
Revenues													5,300,000
Impairment of intangible assets						$ 300,000							$ 300,000