Share-Based Compensation - Summary of Stock Activity under PIP (Detail) (Performance Incentive Plan [Member], USD $)	3 Months Ended
Mar. 31, 2013
Performance Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Outstanding - December 31, 2012	84,645
Granted	23,491
Vested	24,332
Expired	3,043
Number of Shares, Outstanding - Mar 31, 2013	80,761
Weighted Average Fair Value, Outstanding - December 31, 2012	$ 37.86
Weighted Average Fair Value, Granted	$ 44.97
Weighted Average Fair Value, Vested	$ 33.26
Weighted Average Fair Value, Expired	$ 39.12
Weighted Average Fair Value, Outstanding - March 31, 2013	$ 42.14

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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