Debt And Interest Expense (Convertible Debentures) (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Aug. 31, 2007	Dec. 31, 2015	Dec. 31, 2015	Mar. 27, 2015	Dec. 31, 2014	Apr. 16, 2013	Aug. 15, 2007
Debt Disclosure [Abstract]
Principal amount of debt		$ 1,250,000,000	$ 1,250,000,000	$ 500,000,000	$ 1,250,000,000	$ 750,000,000	$ 1,250,000,000
Convertible debenture, interest rate				5.25%		4.625%	3.25%
Convertible debenture, maturity date	Aug. 15, 2037
Conversion rate per 1000 principal amount	2909.68%
Denominator of principal value upon which conversion is based	$ 1,000
Conversion price							$ 34.37
stock reserved for issuance		36.4	36.4
Minimum required sales price as a percentage of conversion price	150.00%
Debt Instrument, Convertible, Stock Price Trigger		$ 44.68
Additional common shares potentially issuable based on if-converted value of convertible debentures			22.1
Debt issuance costs	$ 25,777,000
Discount rate	8.50%		8.50%
Debt Instrument, Convertible, Remaining Discount Amortization Period			21 years 227 days
Effective interest rate							8.39%