Retirement benefits - fair value level 3 transfers (Details) - Pension Plan $ in Millions	12 Months Ended
	Aug. 31, 2024 USD ($)
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Plan assets at fair value at beginning of year	$ 5,594
Plan assets at fair value at end of year	5,994
Annuity policies, less deferred premium 1
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Plan assets at fair value at end of year	5,487
Real estate 2
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Plan assets at fair value at beginning of year	466
Plan assets at fair value at end of year	81
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Plan assets at fair value at beginning of year	1,228
Net realized and unrealized gains/(losses)	248
Net purchases, issuances and Settlement	4,092
Plan assets at fair value at end of year	5,568
Level 3 | Annuity policies, less deferred premium 1
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Plan assets at fair value at beginning of year	48
Net realized and unrealized gains/(losses)	355
Net purchases, issuances and Settlement	5,084
Plan assets at fair value at end of year	5,487
Level 3 | Real estate 2
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Plan assets at fair value at beginning of year	466
Net realized and unrealized gains/(losses)	(52)
Net purchases, issuances and Settlement	(334)
Plan assets at fair value at end of year	81
Level 3 | Direct Private Investment
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Plan assets at fair value at beginning of year	713
Net realized and unrealized gains/(losses)	(55)
Net purchases, issuances and Settlement	(659)
Plan assets at fair value at end of year	$ 0