Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Name:	us-gaap_GoodwillImpairmentLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Miscellaneous Financial Information - Additional Information (Detail) - CAD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Additional Financial Information [Line Items]
Inventory write-down to current market value	$ (281)	$ 281	$ 0	$ 0	$ 0
Upstream [Member]
Additional Financial Information [Line Items]
Non-cash impairment of goodwill		$ 20