Shareholders Equity (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended	3 Months Ended	9 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 At The Market Equity Program [Member]	Sep. 30, 2012 At The Market Equity Program [Member]	May 08, 2012 At The Market Equity Program [Member]	Sep. 30, 2012 Common Stock [Member]	Sep. 30, 2012 5.417% Series 1 Cumulative Convertible Preferred [Member]	Sep. 30, 2011 5.417% Series 1 Cumulative Convertible Preferred [Member]	Sep. 30, 2012 Common Stock [Member]	Sep. 30, 2011 Common Stock [Member]
Preferred Stock, Dividends, Per Share, Cash Paid										$ 1.016	$ 1.016
Preferred Stock, Dividends Per Share, Declared										$ 1.016	$ 1.016
Common Stock, Dividends, Per Share, Declared												$ 2.110	$ 2.030
Common Stock, Dividends, Per Share, Cash Paid												$ 2.070	$ 2.010
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						491,094	873,210		871,629
Weighted average price per common share						$ 107.43	$ 103.54
Net cash proceeds of common stock			95,063,000	153,793,000		52,100,000	89,100,000
Commissions related to sales of common shares						$ 700,000	$ 1,200,000