Retirement benefits - fair value hierarchy (Details) - Pension Plan - USD ($) $ in Millions	Aug. 31, 2020	Aug. 31, 2019	Aug. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	$ 9,614	$ 9,131	$ 8,676
Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	3,200	4,036
Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	5,523	4,288
Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	891	806
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1,505	1,006
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	1,505	1,005
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	515	489
Fixed interest government bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	111	148
Fixed interest government bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	404	341
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	4,168	3,861
Index linked government bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	2,936	3,824
Index linked government bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1,232	37
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	2,730	2,390
Corporate bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1	1
Corporate bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	2,729	2,389
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	492	471
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	492	471
Other investments, net
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	204	914
Other investments, net | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	152	63
Other investments, net | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	(347)	516
Other investments, net | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	$ 399	$ 335