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Financial instruments - Reconciliation of changes in fair value (Details) - Level 3 [member] - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of fair value measurement [line items]    
Opening balance $ 485 $ 713
Total gains and losses recognised in statement of income, assets 4 (122)
Puchases, assets 78 35
Settlement, assets (52) (36)
Transfer to current portion, assets   (88)
Transfer to level 1 (3)  
Foreign currency translation differences (3) (18)
Closing balance 510 485
Non-current derivative financial instruments liabilities [Member]    
Disclosure of fair value measurement [line items]    
Opening balance (35) 0
Total gains and losses recognised in statement of income, liabilities 16 (35)
Puchases, liabilities 0 0
Settlement, liabilities 0 0
Transfer into level 3 0  
Foreign currency translation differences 0 0
Closing balance (19) (35)
Current derivative financial Instruments, liabilities [Member]    
Disclosure of fair value measurement [line items]    
Opening balance (1) (4)
Total gains and losses recognised in statement of income, liabilities 1 3
Puchases, liabilities 0 0
Settlement, liabilities 0 0
Transfer into level 3 0  
Foreign currency translation differences 0 0
Closing balance 0 (1)
Non-current financial investments [Member]    
Disclosure of fair value measurement [line items]    
Opening balance 250 397
Total gains and losses recognised in statement of income, assets (38) (91)
Puchases, assets 78 35
Settlement, assets (11) 0
Transfer to current portion, assets   (88)
Transfer to level 1 (3)  
Foreign currency translation differences 0 (3)
Closing balance 277 250
Non-current derivative financial instruments - assets [Member]    
Disclosure of fair value measurement [line items]    
Opening balance 227 283
Total gains and losses recognised in statement of income, assets (6) (44)
Puchases, assets 0 0
Settlement, assets 0 0
Transfer to current portion, assets   0
Foreign currency translation differences (2) (13)
Closing balance 219 227
Current derivative financial instruments, assets [Member]    
Disclosure of fair value measurement [line items]    
Opening balance 44 37
Total gains and losses recognised in statement of income, assets 31 46
Puchases, assets 0 0
Settlement, assets (42) (36)
Transfer to current portion, assets   0
Foreign currency translation differences 0 (3)
Closing balance $ 33 $ 44