Changes in Accumulated other comprehensive income ("AOCI") by component (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 49,243	$ 43,761	$ 48,890	$ 42,411
Net other comprehensive (loss) income	(1,756)	313	(1,784)	1,031
Ending balance	47,147	44,823	47,147	44,823
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	3,385	1,522	3,413	837
Other comprehensive income (loss) before reclassifications	(1,707)	294	(1,735)	979
Amounts reclassified from AOCI	0	0	0	0
Net other comprehensive (loss) income	(1,707)	294	(1,735)	979
Ending balance	1,678	1,816	1,678	1,816
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	10	6	10	5
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI	1	2	1	3
Net other comprehensive (loss) income	1	2	1	3
Ending balance	11	8	11	8
Pension and post- retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(737)	(1,454)	(738)	(1,463)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI	1	8	2	17
Net other comprehensive (loss) income	1	8	2	17
Ending balance	(736)	(1,446)	(736)	(1,446)
Equity accounted investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(5)	3	(5)	3
Other comprehensive income (loss) before reclassifications	3	(2)	3	(2)
Amounts reclassified from AOCI	0	0	0	0
Net other comprehensive (loss) income	3	(2)	3	(2)
Ending balance	(2)	1	(2)	1
Accumulated other comprehensive income (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	2,653	77	2,680	(618)
Other comprehensive income (loss) before reclassifications	(1,704)	292	(1,732)	977
Amounts reclassified from AOCI	2	10	3	20
Net other comprehensive (loss) income	(1,702)	302	(1,729)	997
Ending balance	$ 951	$ 379	$ 951	$ 379