Stock-Based Compensation - Summary of Total Share Based Liabilities (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share based liabilities paid (in dollars per share)	CAD 48	CAD 90	CAD 29
Deferred Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share based liabilities paid (in dollars per share)	17	3	17
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	79	0
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 0	CAD 8	CAD 12