Divestitures and Acquisitions - Divestitures (Details) - USD ($)			1 Months Ended	9 Months Ended	12 Months Ended
	May 27, 2016	Dec. 22, 2015	Dec. 31, 2015	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Divestitures
Pre-tax gain on sale					$ 21,000	$ 107,699,000
Energy research business | Sale of subsidiaries
Divestitures
Number of years as exclusive sales partner		2 years
After-tax gain on sale of business		$ 91,400,000
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)
Pre-tax gain on sale		$ 107,699,000	$ 107,700,000
ITG Investment Research | Sale of subsidiaries
Divestitures
After-tax gain on sale of business				$ 50,000
Cash proceeds from sale	$ 12,000,000
Carrying value of net assets disposed	(7,502,000)
Direct selling costs	(4,477,000)
Pre-tax gain on sale	$ 21,000