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Selected accounting policies - Additional information (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jul. 01, 2019
Dec. 31, 2019
Jun. 30, 2019
Dec. 31, 2020
Disclosure of expected impact of initial application of new standards or interpretations [line items]        
Cash flow projection period       5 years
Technology-based intangible asset, SAP ERP [member]        
Disclosure of expected impact of initial application of new standards or interpretations [line items]        
Finite-lived intangible asset, useful life 10 years   7 years  
Amortisation decrease due to change in useful life   $ 5   $ 10
Amortisation decrease due to change in useful life, annual amount       $ 10
Bottom of range [member] | Other intangible assets (including software) [member]        
Disclosure of expected impact of initial application of new standards or interpretations [line items]        
Finite-lived intangible asset, useful life   3 years 3 years 3 years
Top of range [member] | Other intangible assets (including software) [member]        
Disclosure of expected impact of initial application of new standards or interpretations [line items]        
Finite-lived intangible asset, useful life   10 years 7 years 10 years