Equity Compensation Plans - Summary of Restricted Stock Activity Under Incentive Plan (Detail) (Restricted Stock [Member], USD $)	3 Months Ended
Mar. 31, 2013
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, Outstanding, Beginning Balance	331,013
Number of Shares, Vested	(49,467)
Number of Shares, Forfeited	(4,615)
Number of Shares, Outstanding, Ending Balance	276,931
Weighted Average Grant-Date Fair Value Per Share, Outstanding, Beginning balance	$ 42.78
Weighted Average Grant Date Fair Value Per Share, Vested	$ 42.30
Weighted Average Grant Date Fair Value Per Share, Forfeited	$ 47.95
Weighted Average Grant Date Fair Value Per Share, Outstanding, Ending balance	$ 42.98

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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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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