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Employee benefit plans - Summary of principal actuarial assumptions (Details)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2020
Dec. 31, 2018
Greece | Employee benefit plans      
Disclosure of defined benefit plans [line items]      
Expected return on plan assets 0.00% 0.00%  
Discount rate - beginning of year 0.90% 0.40% 1.70%
Discount rate - end of year 0.90% 0.40% 1.70%
Rate of salary escalation 2.70% 1.70%  
Turkey | Employee benefit plans      
Disclosure of defined benefit plans [line items]      
Expected return on plan assets 0.00% 0.00%  
Discount rate - beginning of year 13.00% 12.80% 15.00%
Discount rate - end of year 13.00% 12.80% 15.00%
Rate of salary escalation 8.20% 8.50%  
Canada | Employee benefit plans      
Disclosure of defined benefit plans [line items]      
Expected return on plan assets 3.90% 3.10%  
Discount rate - beginning of year 3.10% 0.00% 3.90%
Discount rate - end of year 3.10% 0.00% 3.90%
Rate of salary escalation 2.00% 0.00%  
Average remaining service period of active employees expected to receive benefits 7 months 6 days    
Canada | SERP      
Disclosure of defined benefit plans [line items]      
Expected return on plan assets 3.90% 3.10%  
Discount rate - beginning of year 3.10% 3.10% 3.90%
Discount rate - end of year 3.10% 3.10% 3.90%
Rate of salary escalation 2.00% 0.00%  
Average remaining service period of active employees expected to receive benefits 7 months 6 days