Selected Quarterly Data (Unaudited) (Details) - CAD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 2,069	$ 1,979	$ 1,977	$ 1,767	$ 2,006	$ 1,898	$ 1,750	$ 1,662	$ 7,792	$ 7,316	$ 6,554
Operating income	890	869	822	543	874	790	627	540	3,124	2,831	2,519
Net income	$ 664	$ 618	$ 724	$ 434	$ 545	$ 622	$ 436	$ 348	$ 2,440	$ 1,951	$ 2,405
Basic earnings per share	$ 4.84	$ 4.47	$ 5.19	$ 3.10	$ 3.84	$ 4.36	$ 3.05	$ 2.41	$ 17.58	$ 13.65	$ 16.49
Diluted earnings per share	$ 4.82	$ 4.46	$ 5.17	$ 3.09	$ 3.83	$ 4.35	$ 3.04	$ 2.41	$ 17.52	$ 13.61	$ 16.44