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Assumption Used in Monte Carlo Model to Estimate Fair Value of Performance Shares Awards (Detail) (Performance shares, USD $)
12 Months Ended
Dec. 31, 2011
Year
Performance shares
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Closing price of stock on date of grant $ 38.46
Risk free rate 1.16%
Measurement period 3
Volatility 81.00%