Selected Quarterly Data (Unaudited) (Details) - CAD CAD / shares in Units, 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
Quarterly Financial Information Disclosure [Abstract]
Total revenues	CAD 1,687	CAD 1,709	CAD 1,651	CAD 1,665	CAD 1,760	CAD 1,670	CAD 1,681	CAD 1,509	CAD 6,712	CAD 6,620	CAD 6,133
Operating income	677	753	646	612	708	621	587	423	2,688	2,339	1,420
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
Basic earnings per share (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
Diluted earnings per share (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