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Summary of Nonvested Long-Term Incentive Program Performance Share (Detail) (LTIP Performance Shares, USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
LTIP Performance Shares
     
Number of Shares at Expected Attainment      
Beginning Balance 931,571 499,035 216,150
Granted 273,595 237,751 207,180
Forfeited or expired (103,682) (36,682) (25,620)
Change in expected attainment   231,468 101,325
Ending Balance 1,101,484 931,571 499,035
Weighted-Average Grant Date Fair Value      
Beginning Balance $ 23.33 $ 20.57 $ 16.52
Granted $ 42.65 $ 28.94 $ 26.29
Forfeited or expired $ 23.12 $ 18.92 $ 16.52
Change in expected attainment   $ 22.71 $ 16.52
Ending Balance $ 28.14 $ 23.33 $ 20.57