Shareholder's Equity (Schedule of Stock Option Activity (Details) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Shareholders' Equity
Outstanding at beginning of period, Number of Shares	2,510,054	2,901,989	3,151,052
Options exercised, Number of Shares	(841,599)	(374,350)	(157,750)
Options terminated, Number of Shares	(5,625)	(17,585)	(91,313)
Outstanding at end of period, Number of Shares	1,662,830	2,510,054	2,901,989	3,151,052
Vested or expected to vest at end of period, Number of Shares	1,662,830
Exercisable at end of period, Number of Shares	1,662,830
Outstanding at beginning of period, Weighted Average Exercise Price	$ 23.09	$ 22.49	$ 22.22
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 21.64	$ 18.36	$ 16.38
Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price	$ 21.85	$ 25.42	$ 23.73
Outstanding at end of period, Weighted Average Exercise Price	$ 23.82	$ 23.09	$ 22.49	$ 22.22
Exercisable at end of period, Weighted Average Exercise Price	$ 23.82
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term, Period Start	2 years 8 months	3 years 3 months	4 years 6 months	5 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term, Period End	2 years 8 months	3 years 3 months	4 years 6 months	5 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term	2 years 8 months
Vested or expected to vest at end of period, weighted average exercise price	$ 23.82
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Remaining Contractual Term	2 years 8 months

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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 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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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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The number of exercisable share options (fully vested and expected to vest) that may be converted as of the balance sheet date.

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As of the balance sheet date, the weighted-average exercise price (at which grantees can acquire the shares reserved for issuance) for exercisable stock options that are fully vested or expected to vest.

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Weighted average remaining contractual term for fully vested and expected to vest options that are exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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As of the balance sheet date, the number of shares into which fully vested and expected to vest stock options outstanding can be converted under the option plan.

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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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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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

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