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Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Series B Convertible Preferred Stock |  |  |
Stock issuance costs |  |  | $ 0.1
Initial public offering |  |  |
Stock issuance costs | $ 0.6 | $ 1.1 | $ 7.3

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- Definition The cash outflow for cost incurred directly with the issuance of an equity security. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585 + Details
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