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	1,406	0	0	0	1,406
Other comprehensive income	87	0	0	87	0
Dividends declared	(267)	0	0	0	(267)
Effect of stock-based compensation expense	$ 8	0	8	0	0
Common Shares repurchased (shares)	(2.5)
Common Shares repurchased	$ (559)	(35)	0	0	(524)
Shares issued under stock option plan (shares)	0.2
Shares issued under stock option plan	$ 16	20	(4)	0	0
Ending balance (shares) at Sep. 30, 2018	142.6
Ending balance at Sep. 30, 2018	$ 7,128	2,017	47	(1,654)	6,718
Beginning balance (shares) at Jun. 30, 2018	142.5
Beginning balance at Jun. 30, 2018	$ 6,574	2,013	45	(1,673)	6,189
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	622	0	0	0	622
Other comprehensive income	19	0	0	19	0
Dividends declared	(93)	0	0	0	(93)
Effect of stock-based compensation expense	$ 2	0	2	0	0
Shares issued under stock option plan (shares)	0.1
Shares issued under stock option plan	$ 4	4	0	0	0
Ending balance (shares) at Sep. 30, 2018	142.6
Ending balance at Sep. 30, 2018	$ 7,128	2,017	47	(1,654)	6,718
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	1,776	0	0	0	1,776
Other comprehensive income	51	0	0	51	0
Dividends declared	(320)	0	0	0	(320)
Effect of stock-based compensation expense	$ 11	0	11	0	0
Common Shares repurchased (shares)	(3.2)
Common Shares repurchased	$ (952)	(46)	0	0	(906)
Shares issued under stock option plan (shares)	0.2
Shares issued under stock option plan	$ 18	26	(8)	0	0
Ending balance (shares) at Sep. 30, 2019	137.5
Ending balance at Sep. 30, 2019	$ 7,215	1,982	45	(1,992)	7,180
Beginning balance (shares) at Jun. 30, 2019	139.1
Beginning balance at Jun. 30, 2019	$ 7,157	1,996	45	(2,009)	7,125
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	618	0	0	0	618
Other comprehensive income	17	0	0	17	0
Dividends declared	(114)	0	0	0	(114)
Effect of stock-based compensation expense	$ 3	0	3	0	0
Common Shares repurchased (shares)	(1.6)
Common Shares repurchased	$ (471)	(22)	0	0	(449)
Shares issued under stock option plan (shares)	0.0
Shares issued under stock option plan	$ 5	8	(3)	0	0
Ending balance (shares) at Sep. 30, 2019	137.5
Ending balance at Sep. 30, 2019	$ 7,215	$ 1,982	$ 45	$ (1,992)	$ 7,180