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Debt and Credit Facility - Convertible Senior Notes (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jul. 31, 2010
Debt Instrument [Line Items]        
Proceeds from convertible note hedges $ 0 $ 956 $ 784  
Payments to settle warrants $ 0 469 $ 3,865  
Convertible Senior Notes Due May 2016 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount       $ 1,300
Repayments of Convertible Debt   285    
Conversion value paid in excess of principal   956    
Proceeds from convertible note hedges   $ 956