Changes in Accumulated other comprehensive income ("AOCI") by component (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 44,823	$ 41,004	$ 42,411	$ 38,886
Net other comprehensive (loss) income	(422)	630	609	(13)
Ending balance	45,080	42,255	45,080	42,255
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	1,816	857	837	1,505
Other comprehensive loss before reclassifications	(412)	603	567	(45)
Amounts reclassified from AOCL	0	0	0	0
Net other comprehensive (loss) income	(412)	603	567	(45)
Ending balance	1,404	1,460	1,404	1,460
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	8	2	5	0
Other comprehensive loss before reclassifications	0	0	0	0
Amounts reclassified from AOCL	1	1	4	3
Net other comprehensive (loss) income	1	1	4	3
Ending balance	9	3	9	3
Pension and post- retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(1,446)	(1,406)	(1,463)	(1,410)
Other comprehensive loss before reclassifications	0	0	0	(9)
Amounts reclassified from AOCL	9	6	26	19
Net other comprehensive (loss) income	9	6	26	10
Ending balance	(1,437)	(1,400)	(1,437)	(1,400)
Equity accounted investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	1	3	3	(4)
Other comprehensive loss before reclassifications	(5)	0	(7)	6
Amounts reclassified from AOCL	0	0	0	1
Net other comprehensive (loss) income	(5)	0	(7)	7
Ending balance	(4)	3	(4)	3
Accumulated other comprehensive income (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	379	(544)	(618)	91
Other comprehensive loss before reclassifications	(417)	603	560	(48)
Amounts reclassified from AOCL	10	7	30	23
Net other comprehensive (loss) income	(407)	610	590	(25)
Ending balance	$ (28)	$ 66	$ (28)	$ 66