Stock Plans - Additional Information (Detail) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock-based compensation plans	3
Excess tax benefit from Share-based Compensation	$ 1,412,000	$ 79,000	$ 1,239,000
Stock compensation plan, weighted average grant date fair value of options granted	$ 40.38	$ 40.64
Stock compensation plan, total intrinsic values of options exercised	3,100,000		7,500,000
Stock compensation plan, total intrinsic values of options outstanding	364,000
Stock compensation plan, total intrinsic values of options exercisable	72,800
Stock-based compensation expense	1,482,000	1,047,000	606,000
Stock-based compensation expense, total tax benefit recognized	516,000	359,000	204,000
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1,482,000	1,047,000	606,000
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation plan, stock awards vested	559,000	481,000	362,000
Stock compensation plan, number of restricted stock units outstanding	13,500	8,600
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation plan, conversion ratio of a restricted stock unit to ordinary shares of stock	1
Stock compensation plan, restricted stock units conversion period	5 years
Stock compensation plan, number of restricted stock units outstanding	11,996	17,380
Non Employee Director \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation plan, stock awards vested	22,000	8,000	9,000
Stock compensation plan, number of restricted stock units outstanding	349
Stock compensation plan, total intrinsic value of unvested restricted stock	2,420,000
Stock compensation plan, vested restricted stock total intrinsic value	$ 2,405,000
2006 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation plan, number of common stock shares reserved	200,000
Stock compensation plan, remaining number of common stock shares reserved	59,536
Two Thousand And Six Plan \| Non Employee Director \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation plan, stock awards vested	$ 120,000	$ 120,000
Deferred Compensation Plan \| Non Employee Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation plan, number of common stock shares reserved	2,500
Stock compensation plan, remained shares of common stock reserved for issuance	1,670
Stock Purchase Plan \| Non Employee Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation plan, number of common stock shares reserved	2,500
Stock compensation plan, remaining number of common stock shares reserved	1,126

Number Of Stock Based Compensation Plans

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Share based Compensation Arrangement by Share based Payment Award, Equity Instruments Other than Options, Nonvested, Total Intrinsic Value

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Share Based Compensation Conversion Ratio of Equity Instruments Other Than Option to Ordinary Shares

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Aggregate number of common shares reserved for future issuance.

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Amount of excess tax benefit (tax deficiency) that arises when compensation cost from non-qualified share-based compensation recognized on the entity's tax return exceeds (is less than) compensation cost from equity-based compensation recognized in financial statements. Excess tax benefit (tax deficiency) increases (decreases) net cash provided by financing activities.

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Amount of excess tax benefit (tax deficiency) that arises when compensation cost from non-qualified equity-based compensation recognized on the entity's tax return exceeds (is less than) compensation cost from equity-based compensation recognized in financial statements. Excess tax benefit (tax deficiency) reduces (increases) net cash provided by operating activities.

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The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

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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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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The intrinsic value of equity-based payment equity instruments, excluding stock (or unit) options, that vested during the reporting period as calculated by applying the disclosed pricing methodology.

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The total fair value of equity-based awards 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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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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-Paragraph A240

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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The total accumulated difference between fair values of underlying shares on dates of exercise and exercise price on options which were exercised (or share units converted) into shares during the reporting period under the plan.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of options outstanding.

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