Stockholders' equity (Components of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	$ 10,832	$ 12,500	$ 15,620	$ 12,500	$ 25,241
Foreign currency translation adjustments	(4)		(111)	(17)	(82)
Balance as of end of period	10,794	10,832	14,909	10,794	14,909
Foreign currency translation [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	(683)	(670)		(670)
Foreign currency translation adjustments	(4)	(13)
Income taxes	0	0
Balance as of end of period	(687)	(683)		(687)
Cash flow hedges [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	286	241		241
Unrealized gains	(96)	30
Reclassification adjustments to income	(36)	28
Income taxes	28	(13)
Balance as of end of period	182	286		182
Available-for-sale securities [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	(117)	(338)		(338)
Unrealized gains	161	218
Reclassification adjustments to income	2	4
Income taxes	(10)	(1)
Balance as of end of period	36	(117)		36
Other [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	(2)	(2)		(2)
Unrealized gains	0	0
Other	6
Balance as of end of period	4	(2)		4
AOCI [Member]
Increase (Decrease) in AOCI [Roll Forward]
Balance as of beginning of period	(516)	(769)	(994)	(769)	(679)
Foreign currency translation adjustments	(4)	(13)
Unrealized gains	65	248
Reclassification adjustments to income	(34)	32
Other	6
Income taxes	18	(14)
Balance as of end of period	$ (465)	$ (516)	$ (873)	$ (465)	$ (873)

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount before tax and reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Amount after tax and reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature, attributable to parent entity.

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Amount of tax expense (benefit) allocated to other comprehensive income (loss) attributable to parent entity.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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