Derivative instruments - (Effective Portion of Unrealized Gain (Loss)) (Details) - Cash flow hedge [Member] - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments Gain Loss [Line Items]
Total unrealized gains	$ 127	$ 61	$ 192
Foreign currency contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total unrealized gains	148	348	(402)
Cross-currency swap contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total unrealized gains	(21)	(287)	581
Forward interest rate contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total unrealized gains	$ 0	$ 0	$ 13

Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e637-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109978405&loc=d3e80720-113993

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669646-108580

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration