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Employee benefit plans - Summary of Principal Actuarial Assumptions (Details)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Greece | Employee Benefit Plan    
Disclosure of defined benefit plans [line items]    
Discount rate - beginning of year 1.70% 1.70%
Discount rate - end of year 0.90% 1.70%
Rate of salary escalation 2.70% 2.80%
Turkey | Employee Benefit Plan    
Disclosure of defined benefit plans [line items]    
Discount rate - beginning of year 15.00% 11.00%
Discount rate - end of year 13.00% 15.00%
Rate of salary escalation 8.20% 9.00%
Canada | Employee Benefit Plan    
Disclosure of defined benefit plans [line items]    
Expected return on plan assets 3.90% 3.40%
Discount rate - beginning of year 3.90% 3.40%
Discount rate - end of year 3.10% 3.90%
Rate of salary escalation 2.00% 2.00%
Average remaining service period of active employees expected to receive benefits 7 months 6 days 1 year 7 months 6 days
Canada | SERP    
Disclosure of defined benefit plans [line items]    
Expected return on plan assets 3.90% 3.40%
Discount rate - beginning of year 3.90% 3.40%
Discount rate - end of year 3.10% 3.90%
Rate of salary escalation 2.00% 2.00%
Average remaining service period of active employees expected to receive benefits 7 months 6 days 1 year 7 months 6 days