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Summary of Nonvested Long-Term Incentive Program Performance Share Awards Outstanding and Changes During Period (Detail) (LTIP Performance Shares, USD $)
6 Months Ended
Jun. 30, 2014
LTIP Performance Shares
 
Number of Shares at Expected Attainment  
Beginning Balance 2,718,576
Granted 19,065
Forfeited (67,344)
Vested (635,643)
Change in attainment for 2010 grants 26,226
Ending Balance 2,060,880
Weighted-Average Grant Date Fair Value  
Beginning Balance $ 13.79
Granted $ 20.13
Forfeited $ 15.28
Vested $ 8.88
Change in attainment for 2010 grants $ 8.88
Ending Balance $ 15.25