Summary of Changes in Stock Options Outstanding (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2011 Year
Options
Beginning Balance	2.6
Expired
Exercised	(1.4)
Ending Balance	1.2
Per Share Weighted Average Exercise Price
Beginning Balance	$ 11.13
Expired
Exercised	$ 11.29
Ending Balance	$ 10.93
Weighted Average Remaining Contractual Term
Beginning Balance	2.59
Ending Balance	1.47
Aggregate Intrinsic Value
Beginning Balance	$ 58.2
Exercised	39.6
Ending Balance	$ 27.7

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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 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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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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The combined weighted average exercise price associated with stock options during the period that have either lapsed or been forfeited.

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The total dollar difference between fair values of the underlying shares reserved for issuance and exercise prices pertaining to options outstanding under the plan as of the balance sheet date.

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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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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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The weighted average period between the balance sheet date and expiration for all awards outstanding under the plan, which may be expressed in a decimal value for number of years.

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As of the balance sheet date, the total dollar difference between fair values of the underlying shares reserved for issuance and exercise prices of fully vested and expected to vest options outstanding.

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

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