Stock-based compensation (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	5,892	4,850	5,090	5,271
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 20.55	$ 20.46	$ 20.40	$ 20.37
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	1,761	664	690
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price	$ 19.12	$ 15.70	$ 18.02
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	(523)	(265)	(274)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 13.88	$ 8.43	$ 7.80
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period	(196)	(639)	(597)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price	$ 23.33	$ 20.32	$ 23.13
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	3,328	3,286	3,309
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 22.34	$ 21.59	$ 20.31

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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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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

+ References

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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.

+ References

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

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No definition available.

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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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