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Marketable Securities	12 Months Ended
Marketable Securities	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Marketable Securities	7. Marketable Securities As of both December 31, 2017 and 2016, the Company maintained only cash equivalents comprised of money market funds. As of December 31, 2017, the Company did not hold any marketable securities. There were no realized gains or losses on available-for-sale securities during the years ended December 31, 2017, 2016 or 2015.