Quarterly report pursuant to Section 13 or 15(d)

Share Based Compensation - Summary of Activity Under Share-Based Compensation Plans (Detail)

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Share Based Compensation - Summary of Activity Under Share-Based Compensation Plans (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Awards Outstanding, Beginning Balance 11,333,000 [1],[2]
Granted, Awards 46,000
Exercised, Awards (9,000)
Forfeited, Awards (9,000)
Awards Outstanding, Ending Balance 11,361,000 [1],[2]
Awards Exercisable, Ending Balance 6,235,000 [1]
Weighted Average Exercise Price Outstanding, Beginning Balance $ 28.62
Granted, Weighted Average Exercise Price $ 34.05
Exercised, Weighted Average Exercise Price $ 10.34
Forfeited, Weighted Average Exercise Price $ 44.71
Weighted Average Exercise Price Outstanding, Ending Balance $ 28.65
Exercisable, Weighted Average Exercise Price, Ending Balance $ 17.83
Weighted Average Remaining Contractual Term Outstanding, Beginning Balance 5 years 10 months 24 days
Weighted Average Remaining Contractual Term Outstanding, Ending Balance 5 years 7 months 6 days
Exercisable, Weighted Average Remaining Contractual Term, Ending Balance 3 years 10 months 24 days
Aggregate Intrinsic Value Outstanding, Beginning balance $ 119.1 [3]
Aggregate Intrinsic Value Outstanding, Ending balance 151.6 [3]
Exercisable, Aggregate Intrinsic Value, Ending Balance $ 126.8 [3]
Incentive Plan and Independent Directors Stock Units [Member]
  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Outstanding and nonvested at December 31, 2012, Shares 321,600
Granted, shares 7,600
Vested, Shares (130,500)
Forfeited, Shares   
Outstanding and nonvested at March 31, 2013, Shares 198,700
Outstanding and nonvested at December 31, 2012, Weighted Average Grant Date Fair Value $ 11.70
Granted, Weighted Average Grant Date Fair Value $ 35.63
Vested, Weighted Average Grant Date Fair Value $ 8.16
Forfeited, Weighted Average Grant Date Fair Value   
Outstanding and nonvested at March 31, 2013, Weighted Average Grant Date Fair Value $ 14.93
[1] Includes 1.5 million market condition SARs.
[2] Includes 0.9 million market and performance condition SARs.
[3] The intrinsic value is the amount by which the current market value of the underlying stock exceeds the exercise price of the stock awards.