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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, 2017
Dec. 31, 2016
Defined Benefit Pension Plans [member]    
Investment funds    
Money market 0.00% 2.00%
Canadian fixed income 100.00% 98.00%
Total 100.00% 100.00%
Supplemental pension plan [member]    
Investment funds    
Money market 6.00% 3.00%
Canadian fixed income 2.00% 4.00%
Other 46.00% 44.00%
Total 100.00% 100.00%
Supplemental pension plan [member] | Canada [member]    
Investment funds    
Equities 20.00% 21.00%
Supplemental pension plan [member] | UNITED STATES    
Investment funds    
Equities 19.00% 20.00%
Supplemental pension plan [member] | Other jurisdictions [member]    
Investment funds    
Equities 7.00% 8.00%