Shareholders Equity (Narrative) (Details) (USD $)	9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 At The Market Equity Program [Member]	Sep. 30, 2011 At The Market Equity Program [Member]	Feb. 24, 2011 At The Market Equity Program [Member]
Aggregate offering price of common share					$ 300,000,000
Shares issued under equity program			576,427	1,662,038
Weighted average price per common share			$ 86.73	$ 85.26
Net cash proceeds of common stock	153,793,000	5,997,000	49,200,000	139,300,000
Commissions related to sales of common shares			$ 700,000	$ 2,100,000