Stock-Based Compensation - Summary of Total Share Based Liabilities (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based liabilities paid	$ 58	$ 37	$ 39
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based liabilities paid	54	30	31
Deferred Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based liabilities paid	4	6	6
Other share-based liabilities
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based liabilities paid	$ 0	$ 1	$ 2