Shareholders Equity (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended	3 Months Ended	6 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 At The Market Equity Program [Member]	Jun. 30, 2012 At The Market Equity Program [Member]	May 08, 2012 At The Market Equity Program [Member]	Jun. 30, 2012 Common Stock [Member]	Jun. 30, 2012 5.417% Series 1 Cumulative Convertible Preferred [Member]	Jun. 30, 2011 5.417% Series 1 Cumulative Convertible Preferred [Member]	Jun. 30, 2012 Common Stock [Member]	Jun. 30, 2011 Common Stock [Member]
Preferred Stock, Dividends, Per Share, Cash Paid										$ 0.677	$ 0.677
Preferred Stock, Dividends Per Share, Declared										$ 0.677	$ 0.677
Common Stock, Dividends, Per Share, Declared												$ 1.380	$ 1.340
Common Stock, Dividends, Per Share, Cash Paid												$ 1.380	$ 1.340
Preferred Shares, percentage	5.417%	5.417%	5.417%	5.417%	5.417%
Aggregate offering price of common share								$ 300,000,000
Shares issued under equity program						161,324	382,116		382,194
Weighted average price per common share						$ 101.23	$ 98.54
Net cash proceeds of common stock			40,575,000	96,787,000		16,000,000	37,000,000
Commissions related to sales of common shares						$ 200,000	$ 500,000