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Long-term assets - E.2.1. Accounting for property, plant and equipment (Details)
 12 Months Ended
Dec. 31, 2020
Buildings  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives 40 years
Bottom of range | Networks (including civil works)  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives 5 years
Bottom of range | Other  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives 2 years
Top of range | Networks (including civil works)  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives 15 years
Top of range | Other  
Disclosure of detailed information about property, plant and equipment [line items]  
Estimated useful lives 7 years