Stock Benefit Plans and Stock-Based Compensation - Summary Common Stock Reserved for Future Issuance (Detail) shares in Thousands	Dec. 31, 2017 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Common Shares Issuable Pursuant to Series A Convertible Stock Outstanding	18,274
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Common Shares Issuable Pursuant to Series A Convertible Stock Outstanding	3,156
Unvested restricted share awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Common Shares Issuable Pursuant to Series A Convertible Stock Outstanding	2,000
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Common Shares Issuable Pursuant to Series A Convertible Stock Outstanding	411
Series A Convertible Preferred Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Common Shares Issuable Pursuant to Series A Convertible Stock Outstanding	2,659
Warrants Outstanding
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Common Shares Issuable Pursuant to Series A Convertible Stock Outstanding	9,570
Authorized for Future Grant Under the Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Common Shares Issuable Pursuant to Series A Convertible Stock Outstanding	478