Other comprehensive (loss) income and accumulated other comprehensive (loss) income - Changes in Accumulated Other Comprehensive Loss by Component (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 38,886
Other comprehensive (loss) income before reclassifications	(741)	$ 2,238
Amounts reclassified from AOCI	32	(44)
Net other comprehensive (loss) income	(709)	2,194	$ 711
Ending balance	41,492	38,886
Accumulated other comprehensive (loss) income
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	91	(2,103)
Ending balance	(618)	91	(2,103)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	1,505	(182)
Other comprehensive (loss) income before reclassifications	(668)	1,687
Amounts reclassified from AOCI	0	0
Net other comprehensive (loss) income	(668)	1,687
Ending balance	837	1,505	(182)
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	0	(4)
Other comprehensive (loss) income before reclassifications	0	0
Amounts reclassified from AOCI	5	4
Net other comprehensive (loss) income	5	4
Ending balance	5	0	(4)
Pension and post- retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(1,410)	(1,915)
Other comprehensive (loss) income before reclassifications	(79)	387
Amounts reclassified from AOCI	26	118
Net other comprehensive (loss) income	(53)	505
Ending balance	(1,463)	(1,410)	(1,915)
Equity accounted investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(4)	(2)
Other comprehensive (loss) income before reclassifications	6	164
Amounts reclassified from AOCI	1	(166)
Net other comprehensive (loss) income	7	(2)
Ending balance	$ 3	$ (4)	$ (2)