Stock Option Activity (Detail) (USD $)	3 Months Ended
Mar. 31, 2012
Number of Shares
Options outstanding, January 1, 2012	7,381,329
Granted	1,531,750
Forfeited	(12,063)
Exercised	(775,107)
Options outstanding, March 31, 2012	8,125,909
Weighted Average Exercise Price Per Share
Options outstanding, January 1, 2012	$ 5.22
Granted	$ 14.90
Forfeited	$ 2.89
Exercised	$ 4.23
Options outstanding, March 31, 2012	$ 7.14

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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 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.

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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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Net 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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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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