Consolidated statement of shareholders' equity - CAD ($) $ in Millions	Total	Common shares at stated value (note 11)	Earnings reinvested	Earnings reinvested Cumulative Effect, Period of Adoption, Adjustment [Member]	Accumulated other comprehensive income (loss) (note 18)
At beginning of period at Dec. 31, 2018		$ 1,446	$ 24,560		$ (1,517)
Net income (loss) for the period	$ 1,505		1,505
Share purchases at stated value		(36)
Share purchases in excess of stated value			(693)
Dividends declared			(316)
Other comprehensive income (loss)	73				73
At end of period at Jun. 30, 2019	25,022	1,410	25,056		(1,444)
At beginning of period at Dec. 31, 2018		1,446	24,560		(1,517)
Share purchases at stated value		(71)
Cumulative effect of accounting change (note 5) at Dec. 31, 2019	24,812
At end of period at Dec. 31, 2019	24,276	1,375	24,812		(1,911)
At beginning of period at Mar. 31, 2019		1,427	24,364		(1,472)
Net income (loss) for the period	1,212		1,212
Share purchases at stated value		(17)
Share purchases in excess of stated value			(351)
Dividends declared			(169)
Other comprehensive income (loss)	28				28
At end of period at Jun. 30, 2019	25,022	1,410	25,056		(1,444)
At beginning of period at Dec. 31, 2019	24,276	1,375	24,812		(1,911)
Net income (loss) for the period	(714)		(714)
Share purchases at stated value		(18)
Share purchases in excess of stated value			(256)
Dividends declared			(324)
Cumulative effect of accounting change (note 5) at Jun. 30, 2020	23,516			$ (2)
Other comprehensive income (loss)	(46)				(46)
At end of period at Jun. 30, 2020	22,916	1,357	23,516		(1,957)
At beginning of period at Mar. 31, 2020		1,357	24,204		(1,991)
Net income (loss) for the period	(526)		(526)
Share purchases at stated value		0
Dividends declared			(162)
Cumulative effect of accounting change (note 5) at Jun. 30, 2020	23,516			$ (2)
Other comprehensive income (loss)	34				34
At end of period at Jun. 30, 2020	$ 22,916	$ 1,357	$ 23,516		$ (1,957)