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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 664	$ 618	$ 724	$ 434	$ 545	$ 622	$ 436	$ 348	$ 2,440	$ 1,951	$ 2,405
Weighted-average basic shares outstanding (in shares)									138.8	142.9	145.9
Dilutive effect of weighted average number of stock options (in shares)									0.5	0.4	0.4
Weighted-average diluted shares outstanding (in shares)									139.3	143.3	146.3
Earnings per share - basic	$ 4.84	$ 4.47	$ 5.19	$ 3.10	$ 3.84	$ 4.36	$ 3.05	$ 2.41	$ 17.58	$ 13.65	$ 16.49
Earnings per share - diluted	$ 4.82	$ 4.46	$ 5.17	$ 3.09	$ 3.83	$ 4.35	$ 3.04	$ 2.41	$ 17.52	$ 13.61	$ 16.44