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Estimated Fair Value of Options Granted using Black-Scholes Option-Pricing Model with Weighted-Average Assumptions (Detail)
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2012
Sep. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected life (years) 5 years 6 months 18 days 8 years 4 days 5 years 6 months
Interest rate 0.70% 1.30% 1.60%
Volatility 51.10% 52.40% 50.80%
Dividend yield