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Significant Clients	3 Months Ended
Significant Clients	Mar. 31, 2017
Risks and Uncertainties [Abstract]
Significant Clients	Significant Clients The Company considers significant clients to be those clients who account for 10% or more of the total U.S. equity market making dollar value traded by the Company. No clients accounted for more than 10% of the Company’s U.S. equity dollar value traded during the three months ended March 31, 2017 and 2016.