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Derivative instruments - (Summary of Income and Expense Line Items) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Product sales									$ 25,424	$ 23,362	$ 23,747
Interest and other income, net									256	753	674
Interest expense, net									(1,262)	(1,289)	(1,392)
Interest rate swap agreements [Member]
Derivative [Line Items]
Gains (losses) on fair value hedging relationships, Hedged Items									315	(349)	65
Gains (losses) on fair value hedging relationships, Derivatives designated as hedging instruments									(204)	352	(42)
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Cash flow hedges [Member] \| Cross-currency swap contracts [Member]
Derivative [Line Items]
Interest and other income, net									323	110	(241)
Product sales [Member]
Derivative [Line Items]
Product sales	$ 6,334	$ 6,104	$ 5,908	$ 5,894	$ 5,881	$ 5,463	$ 5,574	$ 5,286	24,240	22,204	22,533
Product sales [Member] \| Reclassification out of Accumulated Other Comprehensive Income [Member] \| Cash flow hedges [Member] \| Foreign currency contracts [Member]
Derivative [Line Items]
Product sales									$ 178	$ 101	$ (21)