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Employee benefits - Average Life Expectancy After Retirement (Details) - Defined benefit retirement plans
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Germany    
Disclosure of defined benefit plans [line items]    
Male 21 years 21 years
Female 24 years 24 years
Sweden    
Disclosure of defined benefit plans [line items]    
Male 22 years 22 years
Female 24 years 24 years
Austria    
Disclosure of defined benefit plans [line items]    
Male 23 years 23 years
Female 25 years 25 years
Italy    
Disclosure of defined benefit plans [line items]    
Male 19 years 19 years
Female 22 years 22 years