Stock-Based Compensation - Summary of Total Share Based Liabilities (Details) - CAD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based liabilities paid		$ 86	$ 58	$ 37
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based liabilities paid	$ 76	76	54	30
Deferred Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based liabilities paid		9	4	6
Other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based liabilities paid		$ 1	$ 0	$ 1