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, 2017		144.9
Beginning balance at Dec. 31, 2017	$ 6,437.0	$ 2,032.0	$ 43.0	$ (1,741.0)	$ 6,103.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	348.0	0.0	0.0	0.0	348.0
Other comprehensive income	36.0	0.0	0.0	36.0	0.0
Dividends declared	(81.0)	0.0	0.0	0.0	(81.0)
Effect of stock-based compensation expense	(4.0)	$ 0.0	(4.0)	0.0	0.0
Common Shares repurchased (shares)		(1.3)
Common Shares repurchased	(318.0)	$ (20.0)	0.0	0.0	(298.0)
Shares issued under stock option plan (shares)		0.1
Shares issued under stock option plan	8.0	$ 10.0	(2.0)	0.0	0.0
Ending balance (shares) at Mar. 31, 2018		143.7
Ending balance at Mar. 31, 2018	6,434.0	$ 2,022.0	45.0	(1,705.0)	6,072.0
Beginning balance (shares) at Dec. 31, 2018		140.5
Beginning balance at Dec. 31, 2018	6,636.0	$ 2,002.0	42.0	(2,043.0)	6,635.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	434.0	0.0	0.0	0.0	434.0
Other comprehensive income	16.0	0.0	0.0	16.0	0.0
Dividends declared	(91.0)	0.0	0.0	0.0	(91.0)
Effect of stock-based compensation expense	(5.0)	$ 0.0	(5.0)	0.0	0.0
Common Shares repurchased (shares)		(0.7)
Common Shares repurchased	(185.0)	$ (10.0)	0.0	0.0	(175.0)
Shares issued under stock option plan (shares)		0.0
Shares issued under stock option plan	4.0	$ 5.0	(1.0)	0.0	0.0
Ending balance (shares) at Mar. 31, 2019		139.8
Ending balance at Mar. 31, 2019	$ 6,814.0	$ 1,997.0	$ 46.0	$ (2,027.0)	$ 6,798.0