Shareholder's Equity (Schedule of Stock Option Activity (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Shareholders' Equity
Outstanding at beginning of period, Number of Shares	2,510,054
Options exercised, Number of Shares	(579,598)
Options terminated, Number of Shares	(5,625)
Outstanding at end of period, Number of Shares	1,924,831	2,510,054
Vested or expected to vest at end of period, Number of Shares	1,924,831
Exercisable at end of period, Number of Shares	1,924,831
Outstanding at beginning of period, Weighted Average Exercise Price	$ 23.09
Options exercised, Weighted Average Exercise Price	$ 20.58
Options terminated, Weighted Average Exercise Price	$ 21.85
Outstanding at end of period, Weighted Average Exercise Price	$ 23.85	$ 23.09
Exercisable at end of period, Weighted Average Exercise Price	$ 23.85
Outstanding at beginning of period, Weighted Average Remaining Contractual Term (in years)	2 years 10 months 24 days	3 years 4 months 24 days
Outstanding at end of period, Weighted Average Remaining Contractual Term (in years)	2 years 10 months 24 days	3 years 4 months 24 days
Vested or expected to vest at beginning of period, Weighted Average Remaining Contractual Term (in years)	2 years 10 months 24 days
Vested or expected to vest at end of period, weighted average exercise price	$ 23.85
Exercisable at end of period, Weighted Average Remaining Contractual Term (in years)	2 years 10 months 24 days