Stockholders' equity (Components of AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 25,241	$ 29,875	$ 28,083
Cumulative effect of changes in accounting principles, net of tax				$ 29
Foreign currency translation adjustments	(141)	81	(99)
Ending Balance	12,500	25,241	29,875
Foreign currency translation [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(529)	(610)	(511)
Foreign currency translation adjustments	(141)	77	(93)
Income taxes	0	4	(6)
Ending Balance	(670)	(529)	(610)
Cash flow hedges [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(6)	282	297
Unrealized gains (losses)	61	192	(176)
Reclassification adjustments to income	262	(638)	139
Income taxes	(76)	158	22
Ending Balance	241	(6)	282
Available-for-sale securities [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(144)	(138)	(260)
Cumulative effect of changes in accounting principles, net of tax				(9)
Unrealized gains (losses)	(556)	(46)	63
Reclassification adjustments to income	365	41	61
Income taxes	6	(1)	(2)
Ending Balance	(338)	(144)	(138)
Other [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	0	(5)	(6)
Other	(2)	5	1
Ending Balance	(2)	0	(5)
AOCI [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(679)	(471)	(480)
Cumulative effect of changes in accounting principles, net of tax				$ (9)
Foreign currency translation adjustments	(141)	77	(93)
Unrealized gains (losses)	(495)	146	(113)
Reclassification adjustments to income	627	(597)	200
Other	(2)	5	1
Income taxes	(70)	161	14
Ending Balance	$ (769)	$ (679)	$ (471)

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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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