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, 2013
LTIP Performance Shares
Number of Shares at Expected Attainment
Beginning Balance	1,101,484
Granted	14,686
Vested	(327,576)
Ending Balance	788,594
Weighted-Average Grant Date Fair Value
Beginning Balance	$ 28.14
Granted	$ 47.63
Vested	$ 16.83
Ending Balance	$ 33.20

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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- Definition

The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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