Retirement benefit obligations - Fair Value of Plan Assets UK and Canada (Details) - USD ($) $ in Millions	Jul. 31, 2024	Jul. 31, 2023	Jul. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 1,308	$ 1,270	$ 1,508
Level 1
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	510	481
Level 2
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	277	279
Level 3
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	521	510	$ 570
Equity securities | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	34	33
Equity securities | Level 1 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	34	33
Equity securities | Level 2 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Equity securities | Level 3 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Corporate | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	340	319
Corporate | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	7	9
Corporate | Level 1 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	1	2
Corporate | Level 1 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Corporate | Level 2 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	227	224
Corporate | Level 2 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	7	9
Corporate | Level 3 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	112	93
Corporate | Level 3 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Asset backed | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	1	1
Asset backed | Level 1 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Asset backed | Level 2 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	1	1
Asset backed | Level 3 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Government | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	439	410
Government | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	33	32
Government | Level 1 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	439	406
Government | Level 1 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Government | Level 2 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	4
Government | Level 2 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	33	32
Government | Level 3 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Government | Level 3 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Cash, cash equivalents and other short-term investments | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	23
Cash, cash equivalents and other short-term investments | Level 1 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	22
Cash, cash equivalents and other short-term investments | Level 2 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	1
Cash, cash equivalents and other short-term investments | Level 3 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0
Cash and cash equivalents | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		29
Cash and cash equivalents | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	2	1
Cash and cash equivalents | Level 1 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		28
Cash and cash equivalents | Level 1 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	2	1
Cash and cash equivalents | Level 2 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		1
Cash and cash equivalents | Level 2 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Cash and cash equivalents | Level 3 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		0
Cash and cash equivalents | Level 3 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Insurance policies | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	409	417
Insurance policies | Level 1 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Insurance policies | Level 2 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Insurance policies | Level 3 | United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	409	417
Other | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	20	19
Other | Level 1 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	12	11
Other | Level 2 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	8	8
Other | Level 3 | Canada
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 0	$ 0