Shareholders' Equity - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 17, 2019	Oct. 19, 2018
Common Shares repurchased	1,000,000.0	1,600,000		2,700,000	3,200,000
Common Shares repurchased (value)	$ 396	$ 471		$ 916	$ 952
Current Normal Course Issuer Bid (NCIB)
Common Shares authorized to be repurchased						4,800,000
Common Shares repurchased	1,044,868		2,950,000	2,651,582
Common Shares repurchased (value)	$ 396		$ 1,016	$ 916
2018 Normal Course Issuer Bid (NCIB)
Common Shares authorized to be repurchased							5,680,000
Common Shares repurchased		1,519,540			3,183,461
Common Shares repurchased (value)		$ 471			$ 952