Quarterly report pursuant to Section 13 or 15(d)

Summary of Unvested Restricted Stock Units (Detail)

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Summary of Unvested Restricted Stock Units (Detail) - Restricted Stock Units (RSUs)
shares in Thousands
9 Months Ended
Sep. 30, 2018
$ / shares
shares
Shares  
Beginning Balance | shares 358
Granted | shares 2,747
Vested | shares (186)
Forfeited or cancelled | shares (338)
Ending Balance | shares 2,581
Weighted Average Grant Date Fair Value  
Unvested RSUs at December 31, 2017 | $ / shares $ 4.84
Granted | $ / shares 1.81
Vested | $ / shares 2.70
Forfeited or cancelled | $ / shares 3.34
Unvested RSUs at September 30, 2018 | $ / shares $ 1.81