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Share-Based Compensation - Weighted Average Contractual Life and Exercise Price (Detail) - Stock Option Plan - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Number of Options    
Options outstanding 3,069,440 3,214,398
Options vested 1,308,203 1,191,368
Options unvested 92,000  
Weighted Average Remaining Contractual Life    
Options outstanding 2 years 4 months 24 days 2 years
Options vested 3 years 3 years
Options unvested 10 months 24 days  
Weighted Average Exercise Price    
Options outstanding $ 4.28 $ 2.68
Options vested 3.94 $ 3.94
Options unvested $ 5.76