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Post-retirement benefits - Additional information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of defined benefit plans [Line Items]      
Securities included in fair value of plan assets $ 13 $ 16  
Defined benefit expenses excluded form unerlying earnings 0 $ 0 $ 0
Estimated contributions to defined benefit pension plans for 2018 $ 210    
Defined benefit obligation weighted average expected future life time 27 years 26 years  
Defined benefit obligation weighted average expected future life time at man aged 60 in 2036 28 years 28 years  
United States [member]      
Disclosure of defined benefit plans [Line Items]      
Discount rate 3.50% 3.90%  
Canada [member]      
Disclosure of defined benefit plans [Line Items]      
Discount rate 3.40% 3.80%  
Healthcare plans [member] | United States [member]      
Disclosure of defined benefit plans [Line Items]      
Discount rate 3.60% 3.90%  
Medical trend rate 9.80% 9.10%  
Reduction in medical trend rate 4.70% 5.10%  
Healthcare plans [member] | Canada [member]      
Disclosure of defined benefit plans [Line Items]      
Discount rate 3.60% 3.90%  
Medical trend rate 9.80% 9.10%  
Reduction in medical trend rate 4.70% 5.10%