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Customer Concentration	12 Months Ended
Customer Concentration	Dec. 31, 2015
Risks And Uncertainties [Abstract]
Customer Concentration	17. Customer Concentration During the years ended December 31, 2015, 2014 and 2013, no single client accounted for more than 10% of total revenue. One institutional investor client accounted for approximately 15.3%, 14.0% and 12.5% of trading volumes during the years ended December 31, 2015, 2014 and 2013, respectively. As of December 31, 2015, investment funds managed by this institutional investor client beneficially owned approximately 8.9% of the outstanding shares of the Company’s common stock, primarily through passive index and ETF funds.