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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net income	CAD 319	CAD 323	CAD 390	CAD 320	CAD 451	CAD 400	CAD 371	CAD 254	CAD 1,352	CAD 1,476	CAD 875
Weighted average basic shares outstanding									159.7	172.8	174.9
Dilutive effect of weighted average number of stock options									1.3	1.6	1.6
Weighted average diluted shares outstanding									161.0	174.4	176.5
Earnings per share - basic (in CAD per share)	CAD 2.09	CAD 2.05	CAD 2.38	CAD 1.94	CAD 2.66	CAD 2.33	CAD 2.13	CAD 1.45	CAD 8.47	CAD 8.54	CAD 5.00
Earnings per share - diluted (in CAD per share)	CAD 2.08	CAD 2.04	CAD 2.36	CAD 1.92	CAD 2.63	CAD 2.31	CAD 2.11	CAD 1.44	CAD 8.40	CAD 8.46	CAD 4.96