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Post-retirement benefits - Summary of main assumptions rate for valuations of plans (Detail)
Dec. 31, 2017
Dec. 31, 2016
Canada [member]    
Disclosure of defined benefit plans [Line Items]    
Discount rate 3.40% 3.80%
Inflation 1.80% 1.90%
Rate of increase in pensions 0.50% 0.40%
Rate of increase in salaries 3.10% 3.20%
United Kingdom [member]    
Disclosure of defined benefit plans [Line Items]    
Discount rate 2.30% 2.60%
Inflation 3.20% 3.30%
Rate of increase in pensions 2.80% 2.90%
Rate of increase in salaries 3.60% 3.70%
United States [member]    
Disclosure of defined benefit plans [Line Items]    
Discount rate 3.50% 3.90%
Inflation 2.10% 2.10%
Rate of increase in pensions 0.00% 0.00%
Rate of increase in salaries 3.60% 3.60%
Switzerland [member]    
Disclosure of defined benefit plans [Line Items]    
Discount rate 0.50% 0.60%
Inflation 1.20% 1.00%
Rate of increase in pensions 0.40% 0.00%
Rate of increase in salaries 2.20% 2.00%
Europe [Member]    
Disclosure of defined benefit plans [Line Items]    
Discount rate 1.50% 1.40%
Inflation 1.70% 1.60%
Rate of increase in pensions 1.70% 1.40%
Rate of increase in salaries 1.90% 2.40%