Other comprehensive income (loss) and accumulated other comprehensive loss - Changes in Accumulated Other Comprehensive Loss by Component (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (2,814)	$ (2,522)
Other comprehensive (loss) income before reclassifications	548	(432)
Amounts reclassified from accumulated other comprehensive loss	163	140
Net other comprehensive income (loss)	711	(292)
AOC Income (Loss), Ending balance	(2,103)	(2,814)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	112	112
Other comprehensive (loss) income before reclassifications	(294)	0
Amounts reclassified from accumulated other comprehensive loss	0	0
Net other comprehensive income (loss)	(294)	0
AOC Income (Loss), Ending balance	(182)	112
Derivatives
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(40)	(47)
Other comprehensive (loss) income before reclassifications	28	(1)
Amounts reclassified from accumulated other comprehensive loss	8	8
Net other comprehensive income (loss)	36	7
AOC Income (Loss), Ending balance	(4)	(40)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(2,878)	(2,580)
Other comprehensive (loss) income before reclassifications	808	(430)
Amounts reclassified from accumulated other comprehensive loss	155	132
Net other comprehensive income (loss)	963	(298)
AOC Income (Loss), Ending balance	(1,915)	(2,878)
Equity accounted investments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(8)	(7)
Other comprehensive (loss) income before reclassifications	6	(1)
Amounts reclassified from accumulated other comprehensive loss	0	0
Net other comprehensive income (loss)	6	(1)
AOC Income (Loss), Ending balance	$ (2)	$ (8)