Summary of Changes in CSEs Outstanding (Detail) (Common Stock Equivalents, USD $)	9 Months Ended
Sep. 30, 2011
Common Stock Equivalents
Common Stock Equivalents (RSUs)
Beginning Balance	117,208
Granted	9,795
Converted to common shares	(12,390)
Forfeited
Ending Balance	114,613
Per Share Weighted Average Grant Date Fair Value
Beginning Balance
Granted	$ 38.13
Converted to common shares
Forfeited
Ending Balance

Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested and Unvested, Conversions in Period

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested and Unvested, Conversions in Period, Weighted Average Grant Date Fair Value

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Share based Compensation Arrangement by Share based Payment Award, Equity Instruments Other than Options, Weighted Average Grant Date Fair Value [Abstract]

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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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The 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 during the reporting period as a result of the occurrence of a terminating event specified in the contractual agreement of the plan.

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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).

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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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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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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