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Summary of Performance Units Activity (Detail) (Performance Units, USD $)
 9 Months Ended
Sep. 30, 2013
Performance Units
  
Number of Share Units  
Outstanding number of Shares (Units), beginning balance 3,041,511
Granted, number of Shares (Units) 2,025,090
Vested, number of shares (Units)   
Forfeited, number of shares (Units) (200,654)
Addition due to achievement of higher than target performance goals, number of shares (Units) 297,911 [1]
Outstanding number of shares (Units), ending balance 5,163,858
Expected to vest 2,542,867 [2]
Weighted Average Grant Date Fair Value  
Outstanding weighted average grant date fair value, beginning balance $ 13.26
Granted, weighted average grant date fair value $ 18.57
Vested, weighted average grant date fair value   
Forfeited, weighted average grant date fair value $ 15.13
Addition due to achievement of higher than target performance goals $ 17.50 [1]
Outstanding weighted Average Grant Date Fair Value, ending balance $ 15.51
Maximum shares eligible to receive  
Outstanding maximum shares eligible to receive, beginning balance 4,402,597
Granted, maximum shares eligible to receive 3,694,635
Vested, maximum shares eligible to receive   
Forfeited, maximum shares eligible to receive (256,064)
Reduction due to achievement of lower than maximum performance goals, maximum shares eligible to receive (373,702) [3]
Outstanding maximum shares eligible to receive, ending balance 7,467,466
[1] Represents the additional award of 22.2% of the target shares on account of the achievement of higher than target performance for the PUs granted in March 2012.
[2] PUs expected to vest are based on the probable achievement of the performance targets after considering an estimated forfeiture rate
[3] Represents a 27.8% reduction in the maximum shares eligible to vest on account of the certification of the performance goals for the PUs granted in 2012.