Changes in Accumulated other comprehensive income (loss) ("AOCI") by component - Changes in accumulated other comprehensive income (loss) (AOCI) by component (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ 91	$ (2,103)
Other comprehensive (loss) income before reclassifications	(24)	(303)
Amounts reclassified from accumulated other comprehensive income	7	33
Net other comprehensive (loss) income	(17)	(270)
AOC Income (Loss), Closing balance	74	(2,373)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	1,505	(182)
Other comprehensive (loss) income before reclassifications	(27)	(349)
Amounts reclassified from accumulated other comprehensive income	0	0
Net other comprehensive (loss) income	(27)	(349)
AOC Income (Loss), Closing balance	1,478	(531)
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	0	(4)
Other comprehensive (loss) income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	1	1
Net other comprehensive (loss) income	1	1
AOC Income (Loss), Closing balance	1	(3)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(1,410)	(1,915)
Other comprehensive (loss) income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	6	31
Net other comprehensive (loss) income	6	31
AOC Income (Loss), Closing balance	(1,404)	(1,884)
Equity accounted investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(4)	(2)
Other comprehensive (loss) income before reclassifications	3	46
Amounts reclassified from accumulated other comprehensive income	0	1
Net other comprehensive (loss) income	3	47
AOC Income (Loss), Closing balance	$ (1)	$ 45