Stock-Based Compensation Plans - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 Year	Jun. 30, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued under the ESPP			20,730	30,699
Incentive plan, weighted-average grant date fair value of stock options granted			$ 13.69	$ 9.39
Incentive plan, total intrinsic value of stock options exercised			$ 3,700,000	$ 900,000
Proceeds from exercises of stock options			2,375,000	2,092,000
Actual tax benefit realized for the tax deductions from option exercises			1,400,000	300,000
Incentive plan, percentage of options expected to vest over the vesting period	94.90%		94.90%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Unrecognized compensation costs	5,600,000		5,600,000
Unrecognized compensation costs weighted-average recognition periods			1.8
Stock-based compensation expenses	2,100,000	1,800,000	4,533,000	3,598,000
Stock-based compensation expenses tax benefits	800,000	700,000	1,700,000	1,300,000
Employee Stock Purchase Plan 1999
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for issuance	1,500,000		1,500,000
Permitted designation for the purchase of common stock under the ESPP			Participating employees are permitted to designate up to the lesser of $25,000 or 10% of their annual base compensation for the purchase of common stock under the ESPP. Purchases under the ESPP are made one calendar month after the end of each fiscal quarter.
Employee participating annual base compensation designated for purchase of common stock, amount	25,000		25,000
Employee participating annual base compensation designated for purchase of common stock, Percent	10.00%		10.00%
Price of common stock purchased under the ESPP			The price for shares of common stock purchased under the ESPP is 85% of the stock’s fair market value on the last business day of the three-month participation period.
Price of common stock purchased under the ESPP, Percent			85.00%
Restricted share awards (RSAs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested Number of Shares, Vested			43,001
Incentive plan, shares withheld to pay employees' portion of the minimum payroll withholding taxes			13,042
Unrecognized compensation costs	1,700,000		1,700,000
Unrecognized compensation costs weighted-average recognition periods			1.4
LTIP Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation costs	$ 6,000,000		$ 6,000,000
Unrecognized compensation costs weighted-average recognition periods			2.1

Base Compensation Designated For Purchase Of Common Stock Maximum, Amount

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Employee Stock Purchase Plan, Purchase Price of Shares

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Employee Stock Purchase Plan Purchase Shares and Contributions By Eligible Participants Description

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Percentage of unvested stock options expected to vest over the remaining vesting period

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As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

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The weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, using a decimal to express in number of years.

+ References

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

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The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

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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 discount rate from market value on purchase date that participants pay for shares.

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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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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

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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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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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- Definition

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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For net-share settlement of share-based awards when the employer settles employees' income tax withholding obligations, this element represents the number of shares the employees use to repay the employer.

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Number of shares issued during the period as a result of an employee stock purchase plan.

+ References

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-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 29, 30
-Article 5

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