Retirement benefits - changes in benefit obligations (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	$ 8,880	$ 9,463
Service costs	5	5	$ 4
Interest costs	193	174	308
Amendments/other	(4)	(11)
Net actuarial gain	(466)	(295)
Benefits paid	(398)	(298)
Currency translation adjustments	83	(158)
Benefit obligation at end of year	$ 8,293	$ 8,880	$ 9,463

Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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Amount of foreign currency translation gain (loss) which (increases) decreases benefit obligation of defined benefit plan.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of increase (decrease) in benefit obligation of defined benefit plan from change in terms of existing plan or initiation of new plan.

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Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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

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