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Summary of Significant Accounting Policies - Anti-dilutive Securities (Details) - shares shares in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Stock options to purchase common stock |  |  |
Anti-dilutive securities excluded from computations of diluted weighted-average common shares outstanding |  |  |
Potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding (in shares) | 9,382 | 8,609 | 7,117
Unvested restricted stock units |  |  |
Anti-dilutive securities excluded from computations of diluted weighted-average common shares outstanding |  |  |
Potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding (in shares) | 228 | 47 | 89
Convertible debt securities |  |  |
Anti-dilutive securities excluded from computations of diluted weighted-average common shares outstanding |  |  |
Potentially dilutive securities excluded from computations of diluted weighted-average shares outstanding (in shares) | 11,492 | 0 | 0

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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://fasb.org/us-gaap/role/ref/legacyRef -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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