Stockholders' equity (Components of AOCI) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 12,500	$ 12,500	$ 25,241	$ 29,875
Cumulative effect of changes in accounting principles, net of taxes					$ 29
Foreign currency translation adjustments		(48)	(141)	81
Ending Balance		9,673	12,500	25,241
Foreign currency translation [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(670)	(670)	(529)	(610)
Foreign currency translation adjustments	(48)		(141)	77
Income taxes	0		0	4
Ending Balance		(718)	(670)	(529)
Cash flow hedges [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	241	241	(6)	282
Unrealized gains	127		61	192
Reclassification adjustments to income	211		262	(638)
Income taxes	18		(76)	158
Ending Balance		175	241	(6)
Available-for-sale securities [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(338)	(338)	(144)	(138)
Cumulative effect of changes in accounting principles, net of taxes					(9)
Unrealized gains	424		(556)	(46)
Reclassification adjustments to income	56		365	41
Income taxes	(8)		6	(1)
Ending Balance		22	(338)	(144)
Other [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(2)	(2)	0	(5)
Other losses		(5)	(2)	5
Ending Balance		(7)	(2)	0
AOCI [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ (769)	(769)	(679)	(471)
Cumulative effect of changes in accounting principles, net of taxes					$ (9)
Foreign currency translation adjustments		(48)	(141)	77
Unrealized gains		551	(495)	146
Reclassification adjustments to income		(267)	627	(597)
Other losses		(5)	(2)	5
Income taxes		10	(70)	161
Ending Balance		$ (528)	$ (769)	$ (679)

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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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