Retirement benefits - Narrative (Details) $ in Millions			3 Months Ended
	Dec. 07, 2023 USD ($)	Nov. 23, 2023 USD ($) member	Nov. 30, 2023 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Accelerated future employer contributions		$ 210
Plan assets, period increase			$ 77
Subsequent Event
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer	$ 375
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Number of members | member		53,000
Minimum
Defined Benefit Plan Disclosure [Line Items]
Expected future employer contributions		$ 760
Maximum
Defined Benefit Plan Disclosure [Line Items]
Expected future employer contributions		$ 820