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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 ($)
Rate
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
Rate
shares
Dec. 31, 2016
USD ($)
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 $ 1,250,000,000
Convertible debenture, interest rate         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     25.4    
Debt issuance costs | $ $ 25,777,000        
Discount rate 8.50%   8.50%    
Debt Instrument, Convertible, Remaining Discount Amortization Period     19 years 227 days    
Effective interest rate         8.39%