Stock-Based Compensation - Summary of Total Share Based Liabilities (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share based liabilities paid (in dollars per share)	CAD 39	CAD 48	CAD 90
Deferred Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share based liabilities paid (in dollars per share)	6	17	3
Performance Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share based liabilities paid (in dollars per share)	31	31	79
Other share based liabilities [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share based liabilities paid (in dollars per share)	CAD 2	CAD 0	CAD 8