Shareholders Equity (Narrative) (Details) (USD $)	12 Months Ended						12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 At The Market Equity Program [Member]	Dec. 31, 2011 At The Market Equity Program [Member]	May 08, 2012 At The Market Equity Program [Member]	Dec. 31, 2012 Common Stock [Member]	Dec. 31, 2011 Common Stock [Member]	Dec. 31, 2010 Common Stock [Member]	Dec. 31, 2012 5.417% Series 1 Cumulative Convertible Preferred [Member]	Dec. 31, 2011 5.417% Series 1 Cumulative Convertible Preferred [Member]	Dec. 31, 2010 5.417% Series 1 Cumulative Convertible Preferred [Member]
Shares issued under dividend reinvestment plan (in shares)	22,814	28,823	34,401				22,814	28,823	34,401
Preferred shares, shares outstanding	399,896	399,896	399,896
Preferred Shares, percentage										5.417%	5.417%	5.417%
Preferred Stock, Liquidation Preference Per Share	$ 25	$ 25	$ 25
Common Stock, Shares, Issued	64,815,446	63,544,150
Preferred shares, par	$ 0.01	$ 0.01	$ 0.01
Convertible Preferred Stock, Converstion Price Per Share	$ 104.69
Aggregate offering price of common share						$ 300,000,000
Shares issued under equity program				1,040,946	1,662,038		1,039,405	1,662,230	190
Weighted average price per common share				$ 103.69	$ 85.26
Net cash proceeds of common stock	114,134,000	156,862,000	6,610,000	106,400,000	139,300,000
Commissions related to sales of common shares				1,400,000	2,100,000
Remaining Capacity To Issue	$ 213,400,000