Selected Quarterly Data (Unaudited) (Details) - CAD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 2,012	$ 1,863	$ 1,792	$ 2,043	$ 2,069	$ 1,979	$ 1,977	$ 1,767	$ 7,710	$ 7,792	$ 7,316
Operating income	928	779	770	834	890	869	822	543	3,311	3,124	2,831
Net income	$ 802	$ 598	$ 635	$ 409	$ 664	$ 618	$ 724	$ 434	$ 2,444	$ 2,440	$ 1,951
Basic earnings per share	$ 5.97	$ 4.42	$ 4.68	$ 2.99	$ 4.84	$ 4.47	$ 5.19	$ 3.10	$ 18.05	$ 17.58	$ 13.65
Diluted earnings per share	$ 5.95	$ 4.41	$ 4.66	$ 2.98	$ 4.82	$ 4.46	$ 5.17	$ 3.09	$ 17.97	$ 17.52	$ 13.61