Retirement benefits - fair value hierarchy (Details) - Pension Plan - USD ($) $ in Millions	Aug. 31, 2019	Aug. 31, 2018	Aug. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	$ 9,131	$ 8,676	$ 8,980
Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	4,036	64
Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	4,288	7,878
Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	806	734
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1,006	1,030
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	1,005	1,030
Equity securities | 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	471	501
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	471	501
Other
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	914	822
Other | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	63	64
Other | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	516	531
Other | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	335	227
Fixed Interest Government Bonds | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	489	901
Fixed Interest Government Bonds | Debt securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	148	0
Fixed Interest Government Bonds | Debt securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	341	901
Fixed Interest Government Bonds | Debt securities | 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 | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	3,861	2,880
Index Linked Government Bonds | Debt securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	3,824	0
Index Linked Government Bonds | Debt securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	37	2,880
Index Linked Government Bonds | Debt securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	0	0
Corporate Bonds | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	2,390	2,542
Corporate Bonds | Debt securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	1	0
Corporate Bonds | Debt securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	2,389	2,536
Corporate Bonds | Debt securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Amount of defined benefit pension plan assets investment of fair market value	$ 0	$ 6