Summary of Nonvested LTIP Performance Shares Outstanding and Changes During Period (Detail) (LTIP Performance Shares, USD $)	3 Months Ended
Mar. 31, 2012
LTIP Performance Shares
Nonvested LTIP Performance Shares
Beginning Balance	931,571
Granted	11,987
Forfeited	(10,426)
Ending Balance	933,132
Weighted-Average Grant Date Fair Value
Beginning Balance	$ 23.33
Granted	$ 39.25
Forfeited	$ 27.22
Ending Balance	$ 23.49

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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