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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income	CAD 384	CAD 347	CAD 328	CAD 540	CAD 319	CAD 323	CAD 390	CAD 320	CAD 1,599	CAD 1,352	CAD 1,476
Weighted average basic shares outstanding (in shares)									149.6	159.7	172.8
Dilutive effect of weighted average number of stock options (in shares)									0.9	1.3	1.6
Weighted average diluted shares outstanding (in shares)									150.5	161.0	174.4
Earnings per share - basic (in CAD per share)	CAD 2.63	CAD 2.35	CAD 2.16	CAD 3.53	CAD 2.09	CAD 2.05	CAD 2.38	CAD 1.94	CAD 10.69	CAD 8.47	CAD 8.54
Earnings per share - diluted (in CAD per share)	CAD 2.61	CAD 2.34	CAD 2.15	CAD 3.51	CAD 2.08	CAD 2.04	CAD 2.36	CAD 1.92	CAD 10.63	CAD 8.40	CAD 8.46