Notes payable and long-term debt - Narrative (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
	Mar. 01, 2023	Sep. 30, 2023	Apr. 01, 2023	Oct. 01, 2022	Sep. 30, 2023	Oct. 01, 2022	Dec. 31, 2022
Debt Instrument [Line Items]
Notional amount of derivative		$ 7,378			$ 7,378		$ 7,372
Unrealized gain (loss) on cash flow hedges, pre-tax		40		$ 62	37	$ 213
Notes Payable, Current		353			353		467
Repayments of Long-term Debt					227	33
Long-term debt		5,530			5,530		5,317
Interest Expense, Debt		83		39	245	149
Current maturities of long-term debt		1,193			$ 1,193		$ 780
WK Kellogg Co
Debt Instrument [Line Items]
Line of Credit Facility, Borrowing Capacity, Description					Under the Credit Facility, WK Kellogg Co has the right at any time, subject to customary conditions, to request incremental term loans or an increase to the revolving credit facility in an aggregate principal amount of (i) up to the greater of (x) $250 million and (y) 100% of Consolidated EBITDA, as defined in the Credit Agreement, for the preceding four fiscal quarters of WK Kellogg Co. Any such addition of or increase in loans will be subject to certain customary conditions precedent and other provisions.
Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Long-Term Line of Credit		164			$ 164
Debt Issuance Costs, Gross		7			7
Current maturities of long-term debt		9			9
Cash Flow Hedging | Interest expense
Debt Instrument [Line Items]
Unrealized gain (loss) on cash flow hedges, pre-tax		$ 14		$ 83	$ 85	$ 165
Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed		2.50%
Debt Instrument, Redemption, Period Two | Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed		5.00%
Debt Instrument, Redemption, Period Three | Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed		5.00%
Debt Instrument, Redemption, Period Four | Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed		7.50%
Debt Instrument, Redemption, Period Five | Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed		10.00%
Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		1.75%			1.75%
Base Rate | Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		0.75%			0.75%
5.25% 10-year U.S. Dollar Notes Due 2033
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 400
Debt Instrument, Interest Rate, Stated Percentage	5.25%
Net proceeds after discount and underwriting commissions	$ 396
Notional amount of derivative	$ 400
Debt Instrument, Interest Rate, Effective Percentage	3.06%
5.25% 10-year U.S. Dollar Notes Due 2033 | Cash Flow Hedging | Interest expense
Debt Instrument [Line Items]
Unrealized gain (loss) on cash flow hedges, pre-tax			$ 47
Cumulative unrealized gain (loss) on cash flow hedge settlement, pre-tax			$ 91
2.75% 10-year U.S. Dollar Notes Due 2023
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 210
Debt Instrument, Interest Rate, Stated Percentage	2.75%
U.S. commercial paper
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		5.44%			5.44%		4.46%
Notes Payable, Current		$ 0			$ 0		$ 330
Bank borrowings
Debt Instrument [Line Items]
Notes Payable, Current		353			353		$ 137
Other assets | Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Debt Issuance Costs, Gross		2			2
Loans | Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Debt Issuance Costs, Gross		5			5
Revolving Credit Facility | Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Long-Term Line of Credit		350			350
Term Loan | Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Long-term debt		500			500
Delayed Draw Term Loan [Member] | Line of Credit | WK Kellogg Co
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		250			250
Long-term debt		$ 250			$ 250
Credit Agreement as a percentage of EBITDA		100.00%			100.00%