Summary of Stock Option Activity (Detail) (USD $)	6 Months Ended	12 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Options Outstanding
Outstanding at beginning of period	2,275,166	2,552,663
Granted
Exercised
Forfeited
Expired	(172,132)	(277,497)
Outstanding at end of period	2,103,034	2,275,166
Weighted-Average Exercise Price
Outstanding at beginning of period	$ 20.67	$ 20.64
Granted
Exercised
Forfeited
Expired	$ 20.03	$ 20.43
Outstanding at end of period	$ 20.72	$ 20.67

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The weighted average price at which option holders acquired shares when converting their stock options into shares under the plan during the reporting period.

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The decrease in the number of shares that could be issued attributable to the lapse of rights to exercise previously issued stock options under the terms of the option agreements under the plan during the reporting period.

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The weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that lapsed during the reporting period.

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The weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated during the reporting period due to noncompliance with plan terms during the reporting period.

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Number of share options (or share units) forfeited during the period.

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Gross number of share options (or share units) granted during the period.

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The weighted average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on stock options awarded under the plan during the reporting period.

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The number of shares reserved for issuance under stock option agreements awarded under the plan that validly exist and are outstanding as of the balance sheet date, including vested options.

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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 price as of the beginning of the year at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Number of share options (or share units) exercised during the current period.

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