Derivative instruments - (Summary of Income and Expense Line Items) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Product sales									$ 23,362	$ 23,747	$ 22,849
Interest and other income, net									753	674	928
Interest expense, net									(1,289)	(1,392)	(1,304)
Interest rate swap agreements [Member]
Derivative [Line Items]
Gains (losses) on fair value hedging relationships, Hedged Items									(349)	65	127
Gains (losses) on fair value hedging relationships, Derivatives designated as hedging instruments									352	(42)	(85)
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									110	(241)	574
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash flow hedges [Member] | Forward interest rate contract [Member]
Derivative [Line Items]
Interest expense, net											(1)
Product sales [Member]
Derivative [Line Items]
Product sales	$ 5,881	$ 5,463	$ 5,574	$ 5,286	$ 6,001	$ 5,510	$ 5,679	$ 5,343	22,204	22,533	21,795
Product sales [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash flow hedges [Member] | Foreign currency contracts [Member]
Derivative [Line Items]
Product sales									$ 101	$ (21)	$ 65