Retirement benefits - fair value hierarchy (Details) - Pension Plan - USD ($) $ in Millions	Aug. 31, 2021	Aug. 31, 2020	Aug. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	$ 10,475	$ 9,614	$ 9,131
Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	3,696	3,200
Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	5,637	5,523
Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1,142	891
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1,316	1,505
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	1,316	1,505
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	514	515
Fixed interest government bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	101	111
Fixed interest government bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	412	404
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	3,521	4,168
Index linked government bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	3,486	2,936
Index linked government bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	35	1,232
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	2,851	2,730
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	2,850	2,729
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	513	492
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	513	492
Other investments, net
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1,761	204
Other investments, net | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	107	152
Other investments, net | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1,024	(347)
Other investments, net | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	$ 629	$ 399