INTERIM CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (unaudited) - CAD ($) shares in Millions, $ in Millions	Total	Share capital	Additional paid-in capital	Accumulated other comprehensive loss	Retained earnings
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 (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
Beginning balance (shares) at Mar. 31, 2018	143.7
Beginning balance at Mar. 31, 2018	$ 6,434	2,022	45	(1,705)	6,072
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	436	0	0	0	436
Other comprehensive income	32	0	0	32	0
Dividends declared	(93)	0	0	0	(93)
Effect of stock-based compensation expense	$ 2	0	2	0	0
Common Shares repurchased (shares)	(1.2)
Common Shares repurchased	$ (241)	(15)	0	0	(226)
Shares issued under stock option plan (shares)	0.0
Shares issued under stock option plan	$ 4	6	(2)	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
Beginning balance (shares) at Dec. 31, 2018	140.5
Beginning balance at Dec. 31, 2018	$ 6,636	2,002	42	(2,043)	6,635
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,158	0	0	0	1,158
Other comprehensive income	34	0	0	34	0
Dividends declared	(206)	0	0	0	(206)
Effect of stock-based compensation expense	$ 8	0	8	0	0
Common Shares repurchased (shares)	(1.6)
Common Shares repurchased	$ (481)	(24)	0	0	(457)
Shares issued under stock option plan (shares)	0.2
Shares issued under stock option plan	$ 13	18	(5)	0	0
Ending balance (shares) at Jun. 30, 2019	139.1
Ending balance at Jun. 30, 2019	$ 7,157	1,996	45	(2,009)	7,125
Beginning balance (shares) at Mar. 31, 2019	139.8
Beginning balance at Mar. 31, 2019	$ 6,814	1,997	46	(2,027)	6,798
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	724	0	0	0	724
Other comprehensive income	18	0	0	18	0
Dividends declared	(115)	0	0	0	(115)
Effect of stock-based compensation expense	$ 3	0	3	0	0
Common Shares repurchased (shares)	(0.9)
Common Shares repurchased	$ (296)	(14)	0	0	(282)
Shares issued under stock option plan (shares)	0.2
Shares issued under stock option plan	$ 9	13	(4)	0	0
Ending balance (shares) at Jun. 30, 2019	139.1
Ending balance at Jun. 30, 2019	$ 7,157	$ 1,996	$ 45	$ (2,009)	$ 7,125