Financial instruments (Details) $ in Millions			3 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
	Jan. 12, 2024 CAD ($)	May 21, 2021 CAD ($)	Dec. 31, 2021 CAD ($)	Mar. 31, 2021 CAD ($)	Jun. 30, 2021 USD ($)	Dec. 31, 2023 CAD ($)	Dec. 31, 2023 CAD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2021 CAD ($)	Dec. 31, 2023 USD ($)	May 21, 2021 USD ($)
Schedule of Investments [Line Items]
Long-term debt, excluding commercial paper						$ 21,437,000,000	$ 21,437,000,000	$ 19,651,000,000
Long-term debt, fair value						$ 20,550,000,000	20,550,000,000	17,720,000,000
FX Forward Contracts
Schedule of Investments [Line Items]
Gain (loss) on derivative							0	0	$ (13,000,000)
Derivative notional amount			$ 1,000,000,000						1,000,000,000
Treasury Rate Locks | Net Interest Expense
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense							$ 7,000,000	6,000,000	10,000,000
Foreign Exchange Contract
Schedule of Investments [Line Items]
Derivative notional amount										$ 215
Derivative, average forward exchange rate						20.61	20.61			20.61
Derivative, loss on derivative						$ 39,000,000
Derivative fair value						60,000,000	$ 60,000,000
Foreign Exchange Contract | 2021 NCIB Program Announcement
Schedule of Investments [Line Items]
Payments for settlement of derivative	$ 65,000,000
Forward starting swaps
Schedule of Investments [Line Items]
Gain (loss) on derivative		$ 251,000,000							(94,000,000)
Derivative notional amount					$ 2,400	1,396,000,000	1,396,000,000	1,745,000,000			$ 2,400
Derivative fair value						$ 116,000,000	116,000,000	(81,000,000)
Derivative term					30 years
AOCI, cash flow hedge, cumulative gain (loss), after tax		73,000,000	(21,000,000)						(21,000,000)
Derivative, gain (loss), settlement			230,000,000
Bond locks
Schedule of Investments [Line Items]
Gain (loss) on derivative									10,000,000
Derivative notional amount		600,000,000		$ 600,000,000
Derivative term				7 years
AOCI, cash flow hedge, cumulative gain (loss), after tax		$ 2,000,000	(17,000,000)						(17,000,000)
Derivative, gain (loss), settlement			$ (7,000,000)
Gain (loss) on derivative, net, subsequent to re-designation									19,000,000
Net Investment Hedge
Schedule of Investments [Line Items]
Gain (loss) on derivative							$ (194,000,000)	$ 471,000,000	$ (25,000,000)