Shareholders' Equity - Activities Under Shares Repurchase Program (Details) - CAD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Common Shares repurchased		3,800,000	4,600,000	1,900,000
Amount of repurchase (in millions)		$ 1,140	$ 1,127	$ 381
2018 and Current Normal Course Issuer Bid
Number of Common Shares repurchased		3,794,149
Weighted-average price per share		$ 300.65
Amount of repurchase (in millions)		$ 1,141
2018 Normal Course Issuer Bid
Number of Common Shares repurchased			4,683,162
Weighted-average price per share			$ 240.68
Amount of repurchase (in millions)			$ 1,127
2017 Normal Course Issuer Bid
Number of Common Shares repurchased				1,888,100
Weighted-average price per share				$ 201.53
Amount of repurchase (in millions)				$ 381
Current Normal Course Issuer Bid
Number of Common Shares repurchased	300,000
Amount of repurchase (in millions)	$ 100