Summary of Significant Accounting Policies - Summary of Basic and Diluted Net Loss per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share, Basic and Diluted [Abstract]
Net loss	$ (8,859)	$ (5,782)	$ (23,534)	$ (20,643)
Earnings Per Share, Basic and Diluted, Other Disclosures [Abstract]
Weighted-average common shares outstanding, basic and diluted (in shares)	42,993	41,386	42,597	41,030
Net loss per common share, basic and diluted (usd per share)	$ (0.21)	$ (0.14)	$ (0.55)	$ (0.50)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	5,052	5,575
Stock options, restricted stock units, and market stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	4,842	5,575
Shares related to the Convertible Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	210	0

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.

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