Selected Quarterly Data (Unaudited) (Details) - CAD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 2,006	$ 1,898	$ 1,750	$ 1,662	$ 1,713	$ 1,595	$ 1,643	$ 1,603	$ 7,316	$ 6,554	$ 6,232
Operating income	874	790	627	540	682	622	611	604	2,831	2,519	2,411
Net income	$ 545	$ 622	$ 436	$ 348	$ 984	$ 510	$ 480	$ 431	$ 1,951	$ 2,405	$ 1,599
Basic earnings per share	$ 3.84	$ 4.36	$ 3.05	$ 2.41	$ 6.79	$ 3.50	$ 3.28	$ 2.94	$ 13.65	$ 16.49	$ 10.69
Diluted earnings per share	$ 3.83	$ 4.35	$ 3.04	$ 2.41	$ 6.77	$ 3.50	$ 3.27	$ 2.93	$ 13.61	$ 16.44	$ 10.63