Shareholders Equity (Narrative) (Details) (USD $)	6 Months Ended		3 Months Ended	6 Months Ended		6 Months Ended	3 Months Ended	6 Months Ended		3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 At The Market Equity Program [Member]	Jun. 30, 2013 At The Market Equity Program [Member]	May 08, 2012 At The Market Equity Program [Member]	Jun. 30, 2013 Common Shares [Member]	Jun. 30, 2013 Commissions [Member] At The Market Equity Program [Member]	Jun. 30, 2013 Commissions [Member] At The Market Equity Program [Member]	Jun. 30, 2013 Other Offering Costs [Member] At The Market Equity Program [Member]	Jun. 30, 2013 less than [Member] Other Offering Costs [Member] At The Market Equity Program [Member]
Aggregate offering price of common share					$ 300,000,000
Shares issued under equity program			432,522	734,579		734,636
Weighted average price per common share			$ 116.55	$ 113.08
Net cash proceeds of common stock	83,220,000	39,629,000	49,800,000	82,000,000
Payments of stock issuance costs							600,000	1,000,000	100,000	100,000
Remaining Capacity To Issue	$ 130,300,000