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Summary of Nonvested LTIP Performance Shares Outstanding and Changes During the Period (Detail) (LTIP Performance Shares, USD $)	6 Months Ended
	Jun. 30, 2011
LTIP Performance Shares
Number of Shares at Expected Attainment
Nonvested Number of Shares, Beginning Balance	499,035
Nonvested Number of Shares, Forfeited	(15,520)
Nonvested Number of Shares, Ending Balance	483,515
Weighted-Average Grant Date Fair Value
Nonvested Weighted-Average Grant Date Fair Value as of Beginning of Period	$ 20.57
Nonvested Weighted-Average Grant Date Fair Value, Forfeited	$ 19.77
Nonvested Weighted-Average Grant Date Fair Value as of end of Period	$ 20.60

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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