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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 545	$ 622	$ 436	$ 348	$ 984	$ 510	$ 480	$ 431	$ 1,951	$ 2,405	$ 1,599
Weighted average basic shares outstanding (in shares)									142.9	145.9	149.6
Dilutive effect of weighted average number of stock options (in shares)									0.4	0.4	0.9
Weighted average diluted shares outstanding (in shares)									143.3	146.3	150.5
Earnings per share - basic (in CAD per share)	$ 3.84	$ 4.36	$ 3.05	$ 2.41	$ 6.79	$ 3.50	$ 3.28	$ 2.94	$ 13.65	$ 16.49	$ 10.69
Earnings per share - diluted (in CAD per share)	$ 3.83	$ 4.35	$ 3.04	$ 2.41	$ 6.77	$ 3.50	$ 3.27	$ 2.93	$ 13.61	$ 16.44	$ 10.63