Retirement benefits - fair value hierarchy (Details) - Pension Plan - USD ($) $ in Millions	Aug. 31, 2024	Aug. 31, 2023	Aug. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	$ 5,994	$ 5,594	$ 6,603
Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	7	2,354
Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	419	2,012
Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	5,568	1,228
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		264
Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		0
Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		264
Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		0
Fixed interest government bonds
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		1,044
Fixed interest government bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		848
Fixed interest government bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		196
Fixed interest government bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		0
Index linked government bonds
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		1,364
Index linked government bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		1,364
Index linked government bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		0
Index linked government bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value		0
Annuity policies, less deferred premium 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	5,487
Annuity policies, less deferred premium 1 | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	0
Annuity policies, less deferred premium 1 | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	0
Annuity policies, less deferred premium 1 | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	5,487	48
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	10	1,214
Corporate bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	0	0
Corporate bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	10	1,214
Corporate bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	0	0
Real estate 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	81	466
Real estate 2 | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	0	0
Real estate 2 | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	0	0
Real estate 2 | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	81	466
Other investments, net
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	416	1,242
Other investments, net | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	7	142
Other investments, net | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	409	338
Other investments, net | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	$ 0	$ 761