Financial instruments - Additional Information (Detail) - CAD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Financial Instruments And Derivatives [Line Items]
Long-term Debt, Fair Value	$ 4,447	$ 4,447

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

No definition available.

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant