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Nonvested Performance Based Share Units and Changes (Detail) (USD $)
6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Share Units    
Nonvested Share Units Beginning Balance 855,134  
Granted 82,150 106,500
Vested (83,480)  
Forfeited (17,500)  
Nonvested Share Units Ending Balance 836,304  
Weighted-Average Grant Date Fair Value    
Nonvested Weighted-Average Grant Date Fair Value Beginning Balance $ 43.25  
Granted $ 109.35  
Vested $ 58.45  
Forfeited $ 51.29  
Nonvested Weighted-Average Grant Date Fair Value Ending Balance $ 48.06