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	911	0	0	0	911
Other comprehensive income	59	0	0	59	0
Dividends declared	(156)	$ 0	0	0	(156)
Common Shares repurchased (in shares)		(0.7)
Common Shares repurchased	(143)	$ (10)	0	0	(133)
Shares issued under stock option plan (shares)		0.5
Shares issued under stock option plan	$ 36	$ 46	(10)	0	0
Ending balance (shares) at Jun. 30, 2017	146.1	146.1
Ending balance at Jun. 30, 2017	$ 5,333	$ 2,038	42	(1,740)	4,993
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	784	0	0	0	784
Other comprehensive income	68	0	0	68	0
Dividends declared	(174)	0	0	0	(174)
Effect of stock-based compensation expense	6	$ 0	6	0	0
Common Shares repurchased (in shares)		(2.5)
Common Shares repurchased	(559)	$ (35)	0	0	(524)
Shares issued under stock option plan (shares)		0.1
Shares issued under stock option plan	$ 12	$ 16	(4)	0	0
Ending balance (shares) at Jun. 30, 2018	142.5
Ending balance at Jun. 30, 2018	$ 6,574	$ 2,013	$ 45	$ (1,673)	$ 6,189