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Summary of Nonvested Long-Term Incentive Program Performance Share Awards Outstanding and Changes During Period (Detail) (LTIP Performance Shares, USD $)
3 Months Ended
Mar. 31, 2014
LTIP Performance Shares
 
Number of Shares at Expected Attainment  
Beginning Balance 906,192
Granted 6,355
Forfeited (12,870)
Vested (211,881)
Change in attainment for 2010 grants 8,742
Ending Balance 696,538
Weighted-Average Grant Date Fair Value  
Beginning Balance $ 41.35
Granted $ 60.40
Forfeited $ 47.14
Vested $ 26.63
Change in attainment for 2010 grants $ 26.63
Ending Balance $ 45.71