Changes in Accumulated other comprehensive income ("AOCI") by component (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 48,890	$ 42,411
Net other comprehensive (loss) income	(28)	718
Ending balance	49,243	43,761
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	3,413	837
Other comprehensive income (loss) before reclassifications	(28)	685
Amounts reclassified from AOCI	0	0
Net other comprehensive (loss) income	(28)	685
Ending balance	3,385	1,522
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	10	5
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from AOCI	0	1
Net other comprehensive (loss) income	0	1
Ending balance	10	6
Pension and post- retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(738)	(1,463)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from AOCI	1	9
Net other comprehensive (loss) income	1	9
Ending balance	(737)	(1,454)
Equity accounted investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(5)	3
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from AOCI	0	0
Net other comprehensive (loss) income	0	0
Ending balance	(5)	3
Accumulated other comprehensive income
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	2,680	(618)
Other comprehensive income (loss) before reclassifications	(28)	685
Amounts reclassified from AOCI	1	10
Net other comprehensive (loss) income	(27)	695
Ending balance	$ 2,653	$ 77