Financial Instruments - Schedule of fair value associated with derivative financial instruments (Details) (Level 2, USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Derivative Assets    
Gross Fair Value $ 1,069 $ 1,093
Impact of Netting (34) (44)
Derivative Liabilities    
Gross Fair Value (35) (45)
Impact of Netting 34 44
Current assets
   
Derivative Assets    
Derivatives instruments 746 752
Non-current assets
   
Derivative Assets    
Derivatives instruments 289 297
Current liabilities
   
Derivative Liabilities    
Derivatives instruments (1) (1)
Non-current liabilities
   
Derivative Liabilities    
Derivatives instruments $ 0 $ 0