Financial instruments - Financial Risk Management (Details) $ in Millions, $ in Billions	3 Months Ended		4 Months Ended		6 Months Ended	12 Months Ended
	Dec. 31, 2021 CAD ($)	Mar. 31, 2021 CAD ($)	Dec. 31, 2021 CAD ($)	Sep. 14, 2021 CAD ($)	Jun. 30, 2021 USD ($)	Dec. 31, 2021 CAD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 CAD ($)	Dec. 31, 2019 CAD ($)	May 21, 2021 CAD ($)	May 21, 2021 USD ($)
Schedule of Investments [Line Items]
Net deferred losses on derivatives and other	$ 4		$ 4			$ 4		$ 40
FX Forward Contracts
Schedule of Investments [Line Items]
Gain (loss) on derivative						13
Derivative entered into, Notional amount							$ 1.0
Derivative notional amount	0		0			0		1,041
Forward starting swaps
Schedule of Investments [Line Items]
Gain (loss) on derivative			94	$ (251)
Derivative term					30 years
Derivative notional amount	2,967		2,967		$ 2.4	2,967		3,540			$ 2.4
Net deferred losses on derivatives and other	(21)		(21)			(21)				$ 73
Derivative, gain (loss), settlement	(230)
Forward Starting Swaps | Net Interest Expense
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense						9		9	$ 9
Net gains (losses) expected to be recognized in next 12 months						(8)
Bond locks
Schedule of Investments [Line Items]
Gain (loss) on derivative			19	$ (10)
Derivative term		7 years
Derivative notional amount		$ 600								600
Net deferred losses on derivatives and other	(17)		$ (17)			(17)				$ 2
Derivative, gain (loss), settlement	$ 7
Treasury Rate Locks | Net Interest Expense
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense						1		1	1
Net gains (losses) expected to be recognized in next 12 months						2
Net Investment Hedge
Schedule of Investments [Line Items]
Gain (loss) on derivative						$ 25		$ 136	$ 288