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Financial instruments and risk management - Summary by currency, Group's net debt, after taking into account relevant cross currency interest rate swaps and foreign exchange contracts (Detail) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Disclosure Of Financial Instruments [Line Items] |  |  |
Total borrowings excluding overdrafts | $ (12,752) | $ (15,176) |
Cash and cash equivalents | 10,773 | 10,550 |
Net cash/(debt) | 255 | (3,845) | $ (9,587)
Foreign exchange risk [member] |  |  |
Disclosure Of Financial Instruments [Line Items] |  |  |
Total borrowings excluding overdrafts | (12,751) |  |
Derivatives related to net debt: designated as hedges | (288) |  |
Cash and cash equivalents | 10,772 |  |
Other investments | 2,522 |  |
Net cash/(debt) | 255 | (3,845) |
Foreign exchange risk [member] | US Dollars [Member] |  |  |
Disclosure Of Financial Instruments [Line Items] |  |  |
Total borrowings excluding overdrafts | (12,080) |  |
Derivatives related to net debt: designated as hedges | (288) |  |
Cash and cash equivalents | 10,310 |  |
Other investments | 2,522 |  |
Net cash/(debt) | 464 | (3,628) |
Foreign exchange risk [member] | Australian dollar [Member] |  |  |
Disclosure Of Financial Instruments [Line Items] |  |  |
Total borrowings excluding overdrafts | (472) |  |
Cash and cash equivalents | 217 |  |
Net cash/(debt) | (255) | (208) |
Foreign exchange risk [member] | Euro [Member] |  |  |
Disclosure Of Financial Instruments [Line Items] |  |  |
Total borrowings excluding overdrafts | (39) |  |
Cash and cash equivalents | 17 |  |
Net cash/(debt) | (22) | (58) |
Foreign exchange risk [member] | South African rand [Member] |  |  |
Disclosure Of Financial Instruments [Line Items] |  |  |
Cash and cash equivalents | 95 |  |
Net cash/(debt) | 95 | 59 |
Foreign exchange risk [member] | Canadian dollar [Member] |  |  |
Disclosure Of Financial Instruments [Line Items] |  |  |
Total borrowings excluding overdrafts | (157) |  |
Cash and cash equivalents | 23 |  |
Net cash/(debt) | (134) | (148) |
Foreign exchange risk [member] | Other currencies [member] |  |  |
Disclosure Of Financial Instruments [Line Items] |  |  |
Total borrowings excluding overdrafts | (3) |  |
Cash and cash equivalents | 110 |  |
Net cash/(debt) | $ 107 | $ 138 |

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