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Summary of Significant Accounting Policies - Net Loss Per Share and Segment Information (Details) | 12 Months Ended
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Dec. 31, 2017 segment shares | Dec. 31, 2016 shares | Dec. 31, 2015 shares
Antidilutive common stock equivalents excluded from computation of diluted net loss per share |  |  |
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares) | 2,870,192 | 2,294,735 | 1,782,959
Segment Information |  |  |
Number of operating segments | segment | 1 |  |
Stock options |  |  |
Antidilutive common stock equivalents excluded from computation of diluted net loss per share |  |  |
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares) | 2,845,626 | 2,270,169 | 1,723,554
Common stock warrants |  |  |
Antidilutive common stock equivalents excluded from computation of diluted net loss per share |  |  |
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares) | 24,566 | 24,566 | 59,405

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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
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