Shareholder's Equity (Schedule of Stock Option Activity (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 Y	Dec. 31, 2010 Y	Dec. 31, 2009 Y
Shareholders' Equity
Outstanding at beginning of period, Number of Shares	2,901,989	3,151,052	3,275,803
Options exercised, Number of Shares	(374,350)	(157,750)	(14,699)
Options terminated, Number of Shares	(17,585)	(91,313)	(110,052)
Outstanding at end of period, Number of Shares	2,510,054	2,901,989	3,151,052
Vested or expected to vest at end of period, Number of Shares	2,510,054
Exercisable at end of period, Number of Shares	2,319,844
Outstanding at beginning of period, Weighted Average Exercise Price	$ 22.49	$ 22.22	$ 22.23
Options exercised, Weighted Average Exercise Price	$ 18.36	$ 16.38	$ 13.67
Options terminated, Weighted Average Exercise Price	$ 25.42	$ 23.73	$ 23.73
Outstanding at end of period, Weighted Average Exercise Price	$ 23.09	$ 22.49	$ 22.22
Exercisable at end of period, Weighted Average Exercise Price	$ 23.02
Outstanding at beginning of period, Weighted Average Remaining Contractual Term (in years)	4.5	5.0	6.7
Outstanding at end of period, Weighted Average Remaining Contractual Term (in years)	3.4	4.5	5.0
Vested or expected to vest at beginning of period, Weighted Average Remaining Contractual Term (in years)	3.4
Vested or expected to vest at end of period, weighted average exercise price	$ 23.09
Exercisable at end of period, Weighted Average Remaining Contractual Term (in years)	3.2