Fair value estimate - Fair value of financial instruments not measured at fair value (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair value of financial instruments
Financial liabilities	$ 23,293	$ 30,410
Debt principal \| Carrying amount
Fair value of financial instruments
Financial liabilities	15,228	21,955
Debt principal \| Fair value
Fair value of financial instruments
Financial liabilities	16,262	23,088
Level 1 \| Debt principal
Fair value of financial instruments
Financial liabilities	10,686	14,935
Level 2 \| Debt principal
Fair value of financial instruments
Financial liabilities	$ 5,576	$ 8,153

- Definition

The amount of liabilities that are: (a) a contractual obligation: (i) to deliver cash or another financial asset to another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially unfavourable to the entity; or (b) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to deliver a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose, rights, options or warrants to acquire a fixed number of the entity’s own equity instruments for a fixed amount of any currency are equity instruments if the entity offers the rights, options or warrants pro rata to all of its existing owners of the same class of its own non-derivative equity instruments. Also, for those purposes the entity’s own equity instruments do not include puttable financial instruments that are classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. As an exception, an instrument that meets the definition of a financial liability is classified as an equity instrument if it has all the features and meets the conditions in paragraphs 16A-16B or paragraphs 16C-16D of IAS 32. [Refer: Financial instruments, class [member]; Financial assets; Derivatives [member]]

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