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Derivative instruments - (Effective Portion of Unrealized Gain (Loss)) (Details) - Cash flow hedge [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Derivative Instruments Gain Loss [Line Items]      
Total unrealized gains $ 127 $ 61 $ 192
Foreign currency contracts [Member]      
Derivative Instruments Gain Loss [Line Items]      
Total unrealized gains 148 348 (402)
Cross-currency swap contracts [Member]      
Derivative Instruments Gain Loss [Line Items]      
Total unrealized gains (21) (287) 581
Forward interest rate contracts [Member]      
Derivative Instruments Gain Loss [Line Items]      
Total unrealized gains $ 0 $ 0 $ 13