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Stock-Based Compensation - Summary of Activity Under Stock Incentive Plans (Detail)
9 Months Ended
Sep. 30, 2017
$ / shares
shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Shares Subject to Outstanding, Beginning Balance | shares 4,578,782
Shares Subject to Outstanding, Options granted | shares 2,180,217
Shares Subject to Outstanding, Options cancelled and forfeited | shares (893,233)
Shares Subject to Outstanding, Ending Balance | shares 5,865,766
Shares Subject to Outstanding, Vested and expected to vest | shares 5,804,209
Shares Subject to Outstanding, Exercisable | shares 2,787,853
Weighted-Average Exercise Price Per Share, Outstanding Beginning Balance | $ / shares $ 4.38
Weighted-Average Exercise Price Per Share, Options granted | $ / shares 0.68
Weighted-Average Exercise Price Per Share, Options cancelled and forfeited | $ / shares 3.42
Weighted Average Exercise Price Per Share, Outstanding Ending Balance | $ / shares 3.15
Weighted Average Exercise Price Per Share, Vested and expected to vest | $ / shares 3.16
Weighted Average Exercise Price Per Share, Exercisable | $ / shares $ 3.91