Activity Under Stock Option Plans (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended
Sep. 30, 2013	Dec. 31, 2011
Number of Shares Underlying Stock Options
Outstanding as of December 31, 2012		3,276
Granted	0
Exercised	(1,430)
Cancelled or expired	(79)
Outstanding as of September 30, 2013	1,767	3,276
Vested and expected to vest at September 30, 2013	1,760
Exercisable at September 30, 2013	1,561
Weighted Average Exercise Price per Share
Outstanding as of September 30, 2013	$ 15.53
Vested and expected to vest at March 31, 2013 (usd per share)	$ 15.51
Exercisable at March 31, 2013 (usd per share)	$ 15.02
Weighted Average Remaining Contractual Term
Outstanding as of September 30, 2013	3 years 9 months 7 days
Vested and expected to vest at September 30, 2013	3 years 9 months 7 days
Exercisable at September 30, 2013	3 years 8 months 27 days
Aggregate Intrinsic Value
Outstanding as of September 30, 2013	$ 57,550
Vested and expected to vest at September 30, 2013	57,368
Exercisable at September 30, 2013	$ 51,635

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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 number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of fully vested and expected to vest options outstanding.

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As of the balance sheet date, the weighted-average exercise price for outstanding stock options that are fully vested or expected to vest.

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