Earnings Per Share - Number Of Shares Used In the Earnings Per Share Calculations (Details) - CAD CAD / shares in Units, shares in Millions, CAD in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	CAD 984	CAD 510	CAD 480	CAD 431	CAD 384	CAD 347	CAD 328	CAD 540	CAD 2,405	CAD 1,599	CAD 1,352
Weighted average basic shares outstanding (in shares)									145.9	149.6	159.7
Dilutive effect of weighted average number of stock options (in shares)									0.4	0.9	1.3
Weighted average diluted shares outstanding (in shares)									146.3	150.5	161.0
Earnings per share - basic (in CAD per share)	CAD 6.79	CAD 3.50	CAD 3.28	CAD 2.94	CAD 2.63	CAD 2.35	CAD 2.16	CAD 3.53	CAD 16.49	CAD 10.69	CAD 8.47
Earnings per share - diluted (in CAD per share)	CAD 6.77	CAD 3.50	CAD 3.27	CAD 2.93	CAD 2.61	CAD 2.34	CAD 2.15	CAD 3.51	CAD 16.44	CAD 10.63	CAD 8.40