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Summary of Status of Outstanding Performance Share Awards (Detail) (Performance shares, USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Performance shares
     
Number of Share      
Beginning Balance 638,870 524,588  
Granted 110,668 141,522 562,182
Forfeited   (27,240) (37,594)
Ending Balance 749,538 638,870 524,588
Weighted Average Grant Date Fair Value      
Beginning Balance $ 13.00 $ 5.64  
Granted $ 57.18 $ 39.36 $ 5.59
Forfeited   $ 8.14 $ 4.94
Ending Balance $ 19.52 $ 13.00 $ 5.64
Aggregate Intrinsic Value      
Aggregate intrinsic value (in thousands) $ 26,691 $ 25,012 $ 14,418