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Fair Values of all Options Granted Estimated Using Binomial Option-Pricing Model with Weighted-average Assumptions (Detail) (USD $)
6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Dividend yield 2.17% 1.72%
Weighted average expected volatility 50.90% 46.87%
Weighted average risk-free interest rate 1.14% 2.58%
Weighted average expected lives (in years) 5 years 4 months 6 days 5 years 8 months 12 days
Weighted average grant-date fair value $ 7.85 $ 7.76