Shareholders Equity (Narrative) (Details) (USD $)	3 Months Ended				3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 At The Market Equity Program [Member]	May 08, 2012 At The Market Equity Program [Member]	Mar. 31, 2013 Common Shares [Member]	Mar. 31, 2013 Commissions [Member] At The Market Equity Program [Member]	Mar. 31, 2013 Other Offering Costs [Member] At The Market Equity Program [Member]
Aggregate offering price of common share				$ 300,000,000
Shares issued under equity program			302,057		302,087
Weighted average price per common share			$ 108.11
Net cash proceeds of common stock	32,542,000	22,402,000	32,200,000
Payments of stock issuance costs						400,000	100,000
Remaining Capacity To Issue	$ 180,700,000