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Debt Long-term Debt Footnote A (Details) - USD ($)
$ in Millions
1 Months Ended
Mar. 31, 2001
Mar. 31, 2016
4.5% U.S. Dollar Notes Due 2046    
Debt Instrument [Line Items]    
Debt Instrument, Face Amount   $ 650
Debt Instrument, Term   30 years
Debt instrument, stated interest rate   4.50%
Debt Instrument, Interest Rate, Effective Percentage   4.59%
7.45% U.S. Dollar Debentures Due 2031    
Debt Instrument [Line Items]    
Debt Instrument, Face Amount $ 625  
Debt Instrument, Term 30 years  
Debt instrument, stated interest rate 7.45%  
Debt Instrument, Interest Rate, Effective Percentage 7.55%