Shareholder's Equity (Schedule of Stock Option Activity (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Shareholders' Equity
Outstanding at beginning of period, Number of Shares	1,662,830
Options exercised, Number of Shares	(236,579)
Outstanding at end of period, Number of Shares	1,426,251	1,662,830
Vested or expected to vest at end of period, Number of Shares	1,426,251
Exercisable at end of period, Number of Shares	1,426,251
Outstanding at beginning of period, Weighted Average Exercise Price	$ 23.82
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 24.06
Outstanding at end of period, Weighted Average Exercise Price	$ 23.78	$ 23.82
Exercisable at end of period, Weighted Average Exercise Price	$ 23.78
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term, Period Start	3 years 0 months 0 days	2 years 10 months 24 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term, Period End	3 years 0 months 0 days	2 years 10 months 24 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term	3 years 0 months 0 days
Vested or expected to vest at end of period, weighted average exercise price	$ 23.78
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Remaining Contractual Term	3 years 0 months 0 days