Summary of Nonvested Restricted Share Awards (Detail) (Restricted share awards (RSAs), USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Restricted share awards (RSAs)
Number of Restricted Shares Awards
Beginning Balance	100,023	192,298	314,548
Granted	56,389	6,300	25,950
Vested	(79,299)	(86,325)	(95,014)
Forfeited or expired	(7,875)	(12,250)	(53,186)
Ending Balance	69,238	100,023	192,298
Weighted-Average Grant Date Fair Value
Beginning Balance	$ 19.29	$ 18.42	$ 17.94
Granted	$ 44.81	$ 28.95	$ 22.19
Vested	$ 18.72	$ 18.31	$ 17.81
Forfeited or expired	$ 16.78	$ 17.52	$ 18.52
Ending Balance	$ 41.02	$ 19.29	$ 18.42

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made 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 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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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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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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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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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