Retirement benefits - narrative (Details) - USD ($) $ in Millions	12 Months Ended
Aug. 31, 2021	Aug. 31, 2020	Aug. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to defined benefit pension plans	$ 41.0
Postretirement Health Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan obligation	154.0	$ 182.0
Expected benefit to be paid net of estimated federal subsidy during fiscal year 2021	9.5
United States
Defined Benefit Plan Disclosure [Line Items]
Profit sharing provision expense	221.0	227.0	$ 239.0
Contributions to profit sharing	222.0	226.0	234.0
Foreign Plan
Defined Benefit Plan Disclosure [Line Items]
Cost recognized in the consolidated condensed statements of earnings	$ 101.0	$ 91.0	$ 101.0

Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

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Amount of benefit for defined benefit plan expected to be paid in next fiscal year following current fiscal year.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
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Amount of cost for defined contribution plan.

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-Publisher FASB
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-SubTopic 70
-Section 50
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Amount of discretionary contributions made by an employer to a defined contribution plan.

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Amount of cash outflow for pension benefit. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree. Excludes other postretirement benefit.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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The amount of the cost recognized during the period for defined contribution plans profit sharing provision benefit expense amount.

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