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Summary of Restricted Stock and Restricted Stock Unit (RSU) Activity (Detail) - Restricted Stock and Restricted Stock Units
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Beginning Balance, Number Outstanding | shares 1,973,459
Granted, Number Outstanding | shares 399,551
Vested, Number Outstanding | shares (702,048)
Forfeited, Number Outstanding | shares (210,918)
Ending Balance, Number Outstanding | shares 1,460,044
Shares vested but not released, Number Outstanding | shares 312,222
Beginning Balance, Weighted Average Fair Value | $ / shares $ 2.80
Granted, Weighted Average Fair Value | $ / shares 4.64
Vested, Weighted Average Fair Value | $ / shares 3.19
Forfeited, Weighted Average Fair Value | $ / shares 3.01
Ending Balance, Weighted Average Fair Value | $ / shares 3.08
Shares vested but not released, Weighted Average Fair Value | $ / shares $ 2.64