Gains and Losses on Asset Sales (Detail) - CAD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Investment And Other Income [Line Items]
Proceeds from asset sales	$ 7	$ 9
Book value of asset sales	4	2
Gain (loss) on asset sales, before-tax	3	7
Gain (loss) on asset sales, after-tax	$ 2	$ 6

Amount of gain (loss) on sale or disposal of assets, including but not limited to property plant and equipment, intangible assets and equity in securities of subsidiaries or equity method investee.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Name:	us-gaap_GainLossOnDispositionOfAssets1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration