Stockholders' Equity - Summary of Common Shares Reserved for Future Issuance (Detail) - shares	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares of common stock reserved for issuance (in shares)	61,708,149	56,805,733
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares of common stock reserved for issuance (in shares)	8,796,212	6,566,483
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares of common stock reserved for issuance (in shares)	454,382	1,055,977
Convertible Notes
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares of common stock reserved for issuance (in shares)	40,649,225	40,649,225
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance or purchase (in shares)	825,537	857,638
Common stock warrants outstanding
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares of common stock reserved for issuance (in shares)	5,815,283	1,886,630
2015 Incentive Compensation Plan | Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance or purchase (in shares)	1,943,085	1,973,537
2018 Omnibus Incentive Compensation Plan | Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance or purchase (in shares)	3,224,425	3,816,243