Retirement benefits - accumulated benefit obligations in excess of plan assets (Details) - Pension Plan - USD ($) $ in Millions	Aug. 31, 2018	Aug. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 8,293	$ 8,880
Accumulated benefit obligation	8,285	8,861
Fair value of plan assets	$ 8,676	$ 8,980

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No definition available.

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Amount of accumulated benefit obligation for defined benefit plan with accumulated benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
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Amount of plan asset for defined benefit plan with accumulated benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Amount of projected benefit obligation for defined benefit pension plan with accumulated benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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