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, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earnings Per Share [Abstract]
Net income	$ 532	$ 472	$ 1,246	$ 602	$ 802	$ 598	$ 635	$ 409	$ 2,852	$ 2,444	$ 2,440
Weighted-average basic shares outstanding (in shares)									679.7	677.2	693.8
Dilutive effect of weighted average number of stock options (in shares)									3.1	2.7	2.5
Weighted-average diluted shares outstanding (in shares)									682.8	679.9	696.3
Earnings per share - basic (cad per share)	$ 0.74	$ 0.71	$ 1.87	$ 0.90	$ 1.19	$ 0.88	$ 0.94	$ 0.60	$ 4.20	$ 3.61	$ 3.52
Earnings per share - diluted (cad per share)	$ 0.74	$ 0.70	$ 1.86	$ 0.90	$ 1.19	$ 0.88	$ 0.93	$ 0.60	$ 4.18	$ 3.59	$ 3.50