Accounting policies -Components of net parent investment (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2023	Sep. 28, 2024	Sep. 30, 2023
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock compensation		$ (11)	$ (3)
Pension and postretirement plan benefit		$ 21	52
Net transfer (to)/from Kellanova	$ 168		74
Kellanova
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Net transfers (to) from Kellanova			(25)
Stock compensation			3
Pension and postretirement plan benefit			(47)
Non-cash contribution of assets and liabilities from parent			143
Net transfer (to)/from Kellanova			$ 74