Shareholders' Equity - Activities Under Shares Repurchase Program (Details) - CAD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Number of Common Shares repurchased	4,000,000.0		3,800,000	4,600,000
Amount of repurchase (in millions)	$ 1,477		$ 1,140	$ 1,127
2019 Normal Course Issuer Bid
Number of Common Shares repurchased	3,973,076	4,270,000
Weighted-average price per share	$ 371.74
Amount of repurchase (in millions)	$ 1,477	$ 1,577
2018 & 2019 Normal Course Issuer Bids
Number of Common Shares repurchased			3,794,149
Weighted-average price per share			$ 300.65
Amount of repurchase (in millions)			$ 1,141
2017 & 2018 Normal Course Issuer Bids
Number of Common Shares repurchased				4,683,162
Weighted-average price per share				$ 240.68
Amount of repurchase (in millions)				$ 1,127