Customer Concentration	12 Months Ended
Customer Concentration	Dec. 31, 2014
Risks And Uncertainties [Abstract]
Customer Concentration	18. Customer Concentration During the years ended December 31, 2014, 2013 and 2012, no single client accounted for more than 10% of total revenue. One institutional investor client accounted for approximately 14.0%, 12.5% and 13.5% of trading volumes during the years ended December 31, 2014, 2013 and 2012, respectively. As of December 31, 2014, investment funds managed by this institutional investor client beneficially owned approximately 9% of the outstanding shares of the Company’s common stock, primarily through passive index and ETF funds.

The entire disclosure for any concentrations existing at the date of the financial statements that make an entity vulnerable to a reasonably possible, near-term, severe impact. This disclosure informs financial statement users about the general nature of the risk associated with the concentration, and may indicate the percentage of concentration risk as of the balance sheet date.

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