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Valuation Assumptions for Stock Options (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate, minimum 1.90% 0.90% 1.00%
Risk-free interest rate, maximum 2.30% 2.20% 2.00%
Expected volatility, minimum 61.00% 55.00% 53.00%
Expected volatility, maximum 67.00% 65.00% 60.00%
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected option life 5 years 8 months 12 days 3 years 6 months 3 years 6 months
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected option life 7 years 2 months 12 days 7 years 6 months 7 years 6 months