Shareholders Equity (Narrative) (Details) (USD $)	0 Months Ended	12 Months Ended					0 Months Ended	12 Months Ended
	Aug. 14, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 At The Market Equity Program [Member]	Feb. 24, 2011 At The Market Equity Program [Member]	Aug. 14, 2009 Common Stock [Member]	Dec. 31, 2011 Common Stock [Member]	Dec. 31, 2010 Common Stock [Member]	Dec. 31, 2009 Common Stock [Member]
Shares issued under dividend reinvestment plan (in shares)		28,823	34,401	50,888				28,823	34,401	50,888
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
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					1,662,038		2,000,000	1,662,230	190	1,995,563
Weighted average price per common share					$ 85.26
Net cash proceeds of common stock	110,000,000	156,862,000	6,610,000	115,502,000	139,300,000
Commissions related to sales of common shares					$ 2,100,000
Common Stock Issued, Per Share	$ 57.50