Common Stock and Stock Option Plan - Classification of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
| Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] | |||
| Stock-based compensation | $ 9,732 | $ 8,237 | $ 495 |
| Research and Development | |||
| Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] | |||
| Stock-based compensation | 4,202 | 4,183 | 124 |
| General and Administrative | |||
| Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] | |||
| Stock-based compensation | $ 5,530 | $ 4,054 | $ 371 |