Debt And Interest Expense (Convertible Debentures) (Narrative) (Details) $ / shares in Units, shares in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Aug. 31, 2007 USD ($) Rate	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) Rate shares	Dec. 31, 2015 USD ($)	Mar. 27, 2015 USD ($) Rate	Apr. 16, 2013 USD ($) Rate	Aug. 15, 2007 USD ($) $ / shares Rate
Debt Disclosure [Abstract]
Principal amount of debt | $		$ 1,250,000,000	$ 1,250,000,000	$ 1,250,000,000	$ 500,000,000	$ 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	29.0968
Denominator of principal value upon which conversion is based | $	$ 1,000
Conversion price | $ / shares							$ 34.37
stock reserved for issuance | shares		36.4	36.4
Minimum required sales price as a percentage of conversion price	150.00%
Debt Instrument, Convertible, Stock Price Trigger | $ / shares		$ 44.68
Additional common shares potentially issuable based on if-converted value of convertible debentures | shares			19.9
Debt issuance costs | $	$ 25,777,000
Discount rate	8.50%		8.50%
Debt Instrument, Convertible, Remaining Discount Amortization Period			20 years 227 days
Effective interest rate							8.39%