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Defined benefit plans - Summary of Defined Benefit Plans' Weighted Average Asset Allocation Percentages by Asset Category (Detail)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Pension Plans [member]    
Investment funds    
Money market 2.00%  
Canadian fixed income 98.00% 100.00%
Total 100.00% 100.00%
Supplemental pension plan [member]    
Investment funds    
Money market 1.00% 6.00%
Canadian fixed income 6.00% 2.00%
Other 49.00% 46.00%
Total 100.00% 100.00%
Supplemental pension plan [member] | Canada [member]    
Investment funds    
Equities 22.00% 20.00%
Supplemental pension plan [member] | UNITED STATES    
Investment funds    
Equities 10.00% 19.00%
Supplemental pension plan [member] | Other jurisdictions [member]    
Investment funds    
Equities 12.00% 7.00%