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Summary of Restricted Stock Unit (RSU) Activity (Detail) - Restricted Stock Units (RSUs)
9 Months Ended
Sep. 30, 2018
$ / shares
shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Unvested beginning balance, Number of RSUs | shares 1,460,044
Granted, Number of RSUs | shares 710,763
Vested, Number of RSUs | shares (457,345)
Forfeited, Number of RSUs | shares (258,417)
Unvested ending balance, Number of RSUs | shares 1,455,045
Shares vested but not released, Number of RSUs | shares 372,018
Unvested beginning balance, Weighted Average Fair Value | $ / shares $ 3.08
Granted, Weighted Average Fair Value | $ / shares 4.02
Vested, Weighted Average Fair Value | $ / shares 3.60
Forfeited, Weighted Average Fair Value | $ / shares 2.67
Unvested ending balance, Weighted Average Fair Value | $ / shares 3.45
Shares vested but not released, Weighted Average Fair Value | $ / shares $ 3.07