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Share Based Compensation - Fair Value of Stock Options Using Black-Scholes Option Pricing Model (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Fair Value Inputs, Equity, Quantitative Information [Line Items]      
Expected stock price volatility 60.20% 56.20% 51.00%
Risk-free interest rate 1.30% 2.40% 2.30%
Expected life of options (years) 6 years 6 years 6 years
Weighted average estimated fair value of options at grant date $ 9.22 $ 10.11 $ 8.98