Income taxes - Reconciliation of Uncertain Tax Positions Related to Unrecognized Tax Benefits (Details) - CAD ($) $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of Uncertain Tax Positions to Unrecognized Tax Benefits [Roll Forward]
Unrecognized tax benefits at January 1		$ 55	$ 52	$ 13
Tax benefits related to the current year		0	0	9
Tax benefits related to prior years	$ 24	0	10	34
Gross uncertain tax benefits related to prior years		(6)	(9)	0
Settlements with taxing authorities		0	2
Settlements with taxing authorities				(4)
Unrecognized tax benefits at December 31	$ 52	$ 49	$ 55	$ 52

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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- Definition

Amount of unrecognized tax benefits.

+ References

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Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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