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	431	0	0	0	431
Other comprehensive income (loss)	30	0	0	30	0
Dividends declared	(73)	0	0	0	(73)
Effect of stock-based compensation expense	(3)	$ 0	(3)	0	0
Shares issued under stock option plan (shares)		0.4
Shares issued under stock option plan	$ 27	$ 34	(7)	0	0
Ending balance (shares) at Mar. 31, 2017	146.7	146.7
Ending balance at Mar. 31, 2017	$ 5,038	$ 2,036	42	(1,769)	4,729
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	348	0	0	0	348
Other comprehensive income (loss)	36	0	0	36	0
Dividends declared	(81)	0	0	0	(81)
Effect of stock-based compensation expense	4	$ 0	4	0	0
Common Shares repurchased (in shares)		(1.3)
Common Shares repurchased	(318)	$ (20)	0	0	(298)
Shares issued under stock option plan (shares)		0.1
Shares issued under stock option plan	$ 8	$ 10	(2)	0	0
Ending balance (shares) at Mar. 31, 2018	143.7	143.7
Ending balance at Mar. 31, 2018	$ 6,434	$ 2,022	$ 45	$ (1,705)	$ 6,072