Retirement benefits - fair value hierarchy (Details) - Pension Plan - USD ($) $ in Millions	Aug. 31, 2023	Aug. 31, 2022	Aug. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	$ 5,594	$ 6,603	$ 10,475
Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	2,354	2,197
Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	2,012	3,145
Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1,228	1,261
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	264	967
Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	0	0
Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	264	967
Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	0	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	688
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	402
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	285
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	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	1,785
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	1,785
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	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	0
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1,214	1,980
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	1,214	1,980
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
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	466	548
Real estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	0	0
Real estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	0	0
Real estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	466	548
Other investments, net
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1,242	636
Other investments, net | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	142	10
Other investments, net | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	338	(87)
Other investments, net | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	$ 761	$ 713