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Summary of Restricted Stock Unit (RSU) Activity (Detail) - Restricted Stock Units (RSUs)
12 Months Ended
Dec. 31, 2018
$ / shares
shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Beginning Balance, Number Outstanding | shares 1,460,044
Granted, Number Outstanding | shares 912,955
Vested, Number Outstanding | shares (606,218)
Forfeited, Number Outstanding | shares (399,151)
Ending Balance, Number Outstanding | shares 1,367,630
Shares vested but not released, Number Outstanding | shares 399,838
Beginning Balance, Weighted Average Fair Value | $ / shares $ 3.08
Granted, Weighted Average Fair Value | $ / shares 4.42
Vested, Weighted Average Fair Value | $ / shares 3.61
Forfeited, Weighted Average Fair Value | $ / shares 2.84
Ending Balance, Weighted Average Fair Value | $ / shares 3.81
Shares vested but not released, Weighted Average Fair Value | $ / shares $ 3.24