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Marketable Securities	9 Months Ended
Marketable Securities	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Marketable Securities	8. Marketable Securities As of both September 30, 2017 and December 31, 2016, the Company maintained only cash equivalents comprised of money market funds. As of September 30, 2017, the Company did not hold any securities that were in an unrealized loss position. There were no realized gains or losses on available-for-sale securities for the three or nine months ended September 30, 2017 or 2016.