Changes in Accumulated other comprehensive (loss) income ("AOCL") by component - Changes in Accumulated Other Comprehensive Income (Loss) (AOCI) by Component (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ 74	$ (2,373)	$ 91	$ (2,103)
Other comprehensive (loss) income before reclassifications	(627)	805	(651)	502
Amounts reclassified from accumulated other comprehensive income	9	29	16	62
Net other comprehensive (loss) income	(618)	834	(635)	564
AOC Income (Loss), Closing balance	(544)	(1,539)	(544)	(1,539)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	1,478	(531)	1,505	(182)
Other comprehensive (loss) income before reclassifications	(621)	748	(648)	399
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Net other comprehensive (loss) income	(621)	748	(648)	399
AOC Income (Loss), Closing balance	857	217	857	217
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	1	(3)	0	(4)
Other comprehensive (loss) income before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income	1	1	2	2
Net other comprehensive (loss) income	1	1	2	2
AOC Income (Loss), Closing balance	2	(2)	2	(2)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(1,404)	(1,884)	(1,410)	(1,915)
Other comprehensive (loss) income before reclassifications	(9)	0	(9)	0
Amounts reclassified from accumulated other comprehensive income	7	28	13	59
Net other comprehensive (loss) income	(2)	28	4	59
AOC Income (Loss), Closing balance	(1,406)	(1,856)	(1,406)	(1,856)
Equity accounted investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(1)	45	(4)	(2)
Other comprehensive (loss) income before reclassifications	3	57	6	103
Amounts reclassified from accumulated other comprehensive income	1	0	1	1
Net other comprehensive (loss) income	4	57	7	104
AOC Income (Loss), Closing balance	$ 3	$ 102	$ 3	$ 102