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Fair value measurements - Fair value and carry value of debt (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Fair Value and Carrying Value of Debt    
Debt, Long-term and Short-term, Combined Amount $ 33,542 $ 26,346
Fair Value, Inputs, Level 2 [Member]    
Fair Value and Carrying Value of Debt    
Debt Instrument, Fair Value Disclosure $ 35,500 $ 27,000