Stock-Based Compensation - Summary of Total Share Based Liabilities (Detail) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based liabilities paid (in dollars per share)	$ 37	$ 39	$ 48
Performance Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based liabilities paid (in dollars per share)	30	31	31
Deferred Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based liabilities paid (in dollars per share)	6	6	17
Other share based liabilities [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based liabilities paid (in dollars per share)	$ 1	$ 2	$ 0