Significant accounting policies - Lease liabilities reconciled to the operating lease commitments (Details) - USD ($) $ in Millions		12 Months Ended
Jan. 01, 2019	Dec. 31, 2019	Dec. 31, 2018
Disclosure of significant accounting policies [Abstract]
Operating lease commitments	$ 459		$ 383
Payments in optional extension periods not recognized as of December 31, 2018			$ 76
Weighted average incremental borrowing rate as of January 1, 2019	7.65%
Discounted operating lease commitments as of January 1, 2019	$ 373
Net adjustment for leases previously classified as finance leases and commitments related to short-term leases and leases of low-value assets	17	$ 28
Lease liabilities	$ 390

The amount of the discounted unguaranteed residual value of assets subject to finance leases. Unguaranteed residual value is the portion of the residual value of the underlying asset, the realisation of which by a lessor is not assured or is guaranteed solely by a party related to the lessor. Finance lease is a lease that transfers substantially all the risks and rewards incidental to ownership of an underlying asset.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 16
-IssueDate 2019-01-01
-Paragraph 94
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2019-en-r&anchor=para_94&doctype=Standard
-URIDate 2019-03-27

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- Definition

The amount of the expense relating to short-term leases accounted for applying paragraph 6 of IFRS 16. This expense need not include the expense relating to leases with a lease term of one month or less. Short-term lease is a lease that, at the commencement date, has a lease term of 12 months or less. A lease that contains a purchase option is not a short-term lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 16
-IssueDate 2019-01-01
-Paragraph 53
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2019-en-r&anchor=para_53_c&doctype=Standard
-URIDate 2019-03-27

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- Definition

The amount of the lease commitments for short-term leases accounted for applying paragraph 6 of IFRS 16. Short-term lease is a lease that, at the commencement date, has a lease term of 12 months or less. A lease that contains a purchase option is not a short-term lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 16
-IssueDate 2019-01-01
-Paragraph 55
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2019-en-r&anchor=para_55&doctype=Standard
-URIDate 2019-03-27

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- Definition

The amount of liabilities related to the entity's leases. Lease is a contract, or part of a contract, that conveys the right to use an underlying asset for a period of time in exchange for consideration.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 16
-IssueDate 2019-01-01
-Paragraph 47
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2019-en-r&anchor=para_47_b&doctype=Standard
-URIDate 2019-03-27

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- Definition

The amount of undiscounted operating lease payments to be received. Operating lease is a lease that does not transfer substantially all the risks and rewards incidental to ownership of an underlying asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 16
-IssueDate 2019-01-01
-Paragraph 97
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2019-en-r&anchor=para_97&doctype=Standard
-URIDate 2019-03-27

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- Definition

The weighted average lessee's incremental borrowing rate applied to lease liabilities recognised in the statement of financial position at the date of initial application of IFRS 16. The incremental borrowing rate is the rate of interest that a lessee would have to pay to borrow over a similar term, and with a similar security, the funds necessary to obtain an asset of a similar value to the right-of-use asset in a similar economic environment.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 16
-IssueDate 2019-01-01
-Paragraph C12
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2019-en-r&anchor=para_C12_a&doctype=Appendix&subtype=C
-URIDate 2019-03-27

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- Definition

Disclosure of significant accounting policies [Abstract]

+ References

No definition available.

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