Summary of major accounting policies - financial instruments (Details)	Aug. 31, 2018
Minimum
Derivative [Line Items]
Highly effective hedging percentage	80.00%
Maximum
Derivative [Line Items]
Highly effective hedging percentage	125.00%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration