Changes in Accumulated other comprehensive income ("AOCI") by component (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 43,761	$ 39,520	$ 42,411	$ 38,886
Net other comprehensive income (loss)	313	(626)	1,031	(643)
Ending balance	44,823	41,004	44,823	41,004
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	1,522	1,478	837	1,505
Other comprehensive income (loss) before reclassifications	294	(621)	979	(648)
Amounts reclassified from AOCI	0	0	0	0
Net other comprehensive income (loss)	294	(621)	979	(648)
Ending balance	1,816	857	1,816	857
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	6	1	5	0
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI	2	1	3	2
Net other comprehensive income (loss)	2	1	3	2
Ending balance	8	2	8	2
Pension and post- retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(1,454)	(1,404)	(1,463)	(1,410)
Other comprehensive income (loss) before reclassifications	0	(9)	0	(9)
Amounts reclassified from AOCI	8	7	17	13
Net other comprehensive income (loss)	8	(2)	17	4
Ending balance	(1,446)	(1,406)	(1,446)	(1,406)
Equity accounted investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	3	(1)	3	(4)
Other comprehensive income (loss) before reclassifications	(2)	3	(2)	6
Amounts reclassified from AOCI	0	1	0	1
Net other comprehensive income (loss)	(2)	4	(2)	7
Ending balance	1	3	1	3
Accumulated other comprehensive Income (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	77	74	(618)	91
Other comprehensive income (loss) before reclassifications	292	(627)	977	(651)
Amounts reclassified from AOCI	10	9	20	16
Net other comprehensive income (loss)	302	(618)	997	(635)
Ending balance	$ 379	$ (544)	$ 379	$ (544)