Stockholders' Equity - Summary of Common Shares Reserved for Future Issuance (Detail) - shares	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares of common stock reserved for issuance (in shares)	56,042,995	61,708,149
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares of common stock reserved for issuance (in shares)	9,044,304	8,796,212
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total shares of common stock reserved for issuance (in shares)	400,315	454,382
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)	622,476	825,537
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)	2,838,454	5,815,283
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)	4,557	1,943,085
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)	2,483,664	3,224,425