Summary of Realized and Unrealized Gain or (Loss) on Derivative Instruments (Detail) - CAD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Financial Instruments And Derivatives [Line Items]
Revenues	$ 0	$ 1
Purchases of crude oil and products	(14)	34
Total	$ (14)	$ 35

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5618551-113959

Name:	us-gaap_GainLossOnDerivativeInstrumentsNetPretax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration