INTERIM CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (unaudited) - CAD ($) shares in Millions, $ in Millions	Total	Share capital [Member]	Additional paid-in capital [Member]	Accumulated other comprehensive loss [Member]	Retained earnings [Member]
Beginning balance (shares) at Dec. 31, 2016		146.3
Beginning balance at Dec. 31, 2016	$ 4,626	$ 2,002	$ 52	$ (1,799)	$ 4,371
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,421	0	0	0	1,421
Other comprehensive income	84	0	0	84	0
Dividends declared	(237)	$ 0	0	0	(237)
Common Shares repurchased (in shares)		(1.8)
Common Shares repurchased	(368)	$ (26)	0	0	(342)
Shares issued under stock option plan (shares)		0.5
Shares issued under stock option plan	39	$ 49	(10)	0	0
Ending balance (shares) at Sep. 30, 2017		145.0
Ending balance at Sep. 30, 2017	5,565	$ 2,025	42	(1,715)	5,213
Beginning balance (shares) at Dec. 31, 2017		144.9
Beginning balance at Dec. 31, 2017	6,437	$ 2,032	43	(1,741)	6,103
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,406	0	0	0	1,406
Other comprehensive income	87	0	0	87	0
Dividends declared	(267)	0	0	0	(267)
Effect of stock-based compensation expense	8	$ 0	8	0	0
Common Shares repurchased (in shares)		(2.5)
Common Shares repurchased	(559)	$ (35)	0	0	(524)
Shares issued under stock option plan (shares)		0.2
Shares issued under stock option plan	16	$ 20	(4)	0	0
Ending balance (shares) at Sep. 30, 2018		142.6
Ending balance at Sep. 30, 2018	$ 7,128	$ 2,017	$ 47	$ (1,654)	$ 6,718