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Weighted-Average Assumptions for Stock Option Grants using Black-Scholes Option-Pricing Model (Detail) - Stock Options
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Risk-free interest rate 1.90% 1.50% 2.00% 1.40%
Expected volatility of common stock 58.50% 60.00% 58.10% 60.00%
Dividend yield 0.00% 0.00% 0.00% 0.00%
Expected option term 5 years 7 months 6 days 5 years 8 months 12 days 5 years 8 months 12 days 5 years 7 months 6 days