Shareholders' Equity - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 19, 2020	Jan. 29, 2021	Dec. 20, 2019
Common Shares repurchased		1,600,000
Common Shares repurchased (value)		$ 468
2021 Normal Course Issuer Bid (NCIB)
Common Shares authorized to be repurchased				3,330,000
Common Shares repurchased	0
Common Shares repurchased (value)	$ 0
2019 Normal Course Issuer Bid (NCIB)
Common Shares authorized to be repurchased					4,800,000
Common Shares repurchased		1,455,854	4,270,000
Common Shares repurchased (value)		$ 468	$ 1,577