Changes in Accumulated other comprehensive income ("AOCL") by component - Changes in Accumulated Other Comprehensive Income (Loss) (AOCI) by Component (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (544)	$ (1,539)	$ 91	$ (2,103)
Other comprehensive income before reclassifications	603	1,641	(48)	2,143
Amounts reclassified from accumulated other comprehensive income	7	31	23	93
Net other comprehensive income	610	1,672	(25)	2,236
AOC Income (Loss), Closing balance	66	133	66	133
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	857	217	1,505	(182)
Other comprehensive income before reclassifications	603	1,618	(45)	2,017
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Net other comprehensive income	603	1,618	(45)	2,017
AOC Income (Loss), Closing balance	1,460	1,835	1,460	1,835
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	2	(2)	0	(4)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income	1	1	3	3
Net other comprehensive income	1	1	3	3
AOC Income (Loss), Closing balance	3	(1)	3	(1)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(1,406)	(1,856)	(1,410)	(1,915)
Other comprehensive income before reclassifications	0	(14)	(9)	(14)
Amounts reclassified from accumulated other comprehensive income	6	30	19	89
Net other comprehensive income	6	16	10	75
AOC Income (Loss), Closing balance	(1,400)	(1,840)	(1,400)	(1,840)
Equity accounted investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	3	102	(4)	(2)
Other comprehensive income before reclassifications	0	37	6	140
Amounts reclassified from accumulated other comprehensive income	0	0	1	1
Net other comprehensive income	0	37	7	141
AOC Income (Loss), Closing balance	$ 3	$ 139	$ 3	$ 139