INTERIM CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (unaudited) - CAD shares in Millions, CAD in Millions	Total	Common shares/Share capital [Member]	Additional paid-in capital [Member]	Accumulated other comprehensive loss [Member]	Retained earnings [Member]
Beginning balance (shares) at Dec. 31, 2015		153.0
Beginning balance at Dec. 31, 2015	CAD 4,796	CAD 2,058	CAD 43	CAD (1,477)	CAD 4,172
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	540	0	0	0	540
Other comprehensive (loss) income	(4)	0	0	(4)	0
Dividends declared	(54)	0	0	0	(54)
Effect of stock-based compensation expense	6	CAD 0	6	0	0
Shares issued under stock option plan (shares)		0.0
Shares issued under stock option plan	CAD 6	CAD 7	(1)	0	0
Ending balance (shares) at Mar. 31, 2016	153.0	153.0
Ending balance at Mar. 31, 2016	CAD 5,290	CAD 2,065	48	(1,481)	4,658
Beginning balance (shares) at Dec. 31, 2016		146.3
Beginning balance at Dec. 31, 2016	4,626	CAD 2,002	52	(1,799)	4,371
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	431	0	0	0	431
Other comprehensive (loss) income	30	0	0	30	0
Dividends declared	(73)	0	0	0	(73)
Effect of stock-based compensation expense	(3)	CAD 0	(3)	0	0
Shares issued under stock option plan (shares)		0.4
Shares issued under stock option plan	CAD 27	CAD 34	(7)	0	0
Ending balance (shares) at Mar. 31, 2017	146.7	146.7
Ending balance at Mar. 31, 2017	CAD 5,038	CAD 2,036	CAD 42	CAD (1,769)	CAD 4,729