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Summary of Nonvested LTIP Performance Shares (Detail) (LTIP Performance Shares, USD $)
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
LTIP Performance Shares
     
Nonvested LTIP Performance Shares      
Beginning Balance 499,035 216,150  
Granted 237,751 207,180 216,150
Forfeited or expired (36,682) (25,620)  
Change in expected attainment 231,468 101,325  
Ending Balance 931,571 499,035 216,150
Weighted-Average Grant Date Fair Value      
Beginning Balance $ 20.57 $ 16.52  
Granted $ 28.94 $ 26.29 $ 16.52
Forfeited or expired $ 18.92 $ 16.52  
Change in expected attainment $ 22.71 $ 16.52  
Ending Balance $ 23.33 $ 20.57 $ 16.52