Financial and derivative instruments - Summary of estimated fair value of financial instruments (Detail) - CAD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020
Derivative Asset [Abstract]
Derivative Asset	[1]	$ 7	$ 2
Derivative Liability [Abstract]
Derivative liabilities	[2]	2	12
Effect of counterparty netting [Member]
Derivative Asset [Abstract]
Derivative Asset	[1]	(2)	(2)
Derivative Liability [Abstract]
Derivative liabilities	[2]	(2)	(2)
Derivative fair value of derivative amount effect of collateral netting [Member]
Derivative Asset [Abstract]
Derivative Asset	[1]	(5)	0
Derivative Liability [Abstract]
Derivative liabilities	[2]	0	(10)
Fair Value, Inputs, Level 1 [Member]
Derivative Asset [Abstract]
Derivative Asset	[1]	7	2
Derivative Liability [Abstract]
Derivative liabilities	[2]	2	12
Fair Value, Inputs, Level 2 [Member]
Derivative Asset [Abstract]
Derivative Asset	[1]	0	0
Derivative Liability [Abstract]
Derivative liabilities	[2]	0	0
Fair Value, Inputs, Level 3 [Member]
Derivative Asset [Abstract]
Derivative Asset	[1]	0	0
Derivative Liability [Abstract]
Derivative liabilities	[2]	$ 0	$ 0

[1]  Included in the Consolidated balance sheet line: “Materials, supplies and prepaid expenses”.

[2]  Included in the Consolidated balance sheet line: “Accounts payable and accrued liabilities”.