Earnings Per Share - Number Of Shares Used In the Earnings Per Share Calculations (Details) - CAD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 802	$ 598	$ 635	$ 409	$ 664	$ 618	$ 724	$ 434	$ 2,444	$ 2,440	$ 1,951
Weighted-average basic shares outstanding									135.5	138.8	142.9
Dilutive effect of weighted average number of stock options									0.5	0.5	0.4
Weighted-average diluted shares outstanding									136.0	139.3	143.3
Earnings per share - basic	$ 5.97	$ 4.42	$ 4.68	$ 2.99	$ 4.84	$ 4.47	$ 5.19	$ 3.10	$ 18.05	$ 17.58	$ 13.65
Earnings per share - diluted	$ 5.95	$ 4.41	$ 4.66	$ 2.98	$ 4.82	$ 4.46	$ 5.17	$ 3.09	$ 17.97	$ 17.52	$ 13.61