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Customer Concentration	12 Months Ended
Customer Concentration	Dec. 31, 2017
Risks And Uncertainties [Abstract]
Customer Concentration	15. Customer Concentration During the years ended December 31, 2017, 2016 and 2015, no single client accounted for more than 10% of total revenue. One institutional investor client accounted for approximately 12.2%, 14.2% and 15.3% of trading volumes during the years ended December 31, 2017, 2016 and 2015, respectively. As of December 31, 2017, investment funds managed by this institutional investor client beneficially owned approximately 8.3% of the outstanding shares of the Company’s common stock, primarily through passive index and ETF funds.