Divestiures, West Africa Investments and Acquisitions Multipro Schedule of Pro Forma Results Narrative (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Jan. 02, 2021	Sep. 26, 2020	Jun. 27, 2020	Mar. 28, 2020	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Jun. 30, 2018	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Business Acquisition [Line Items]
Net sales	$ 3,464	$ 3,429	$ 3,465	$ 3,412	$ 3,223	$ 3,372	$ 3,461	$ 3,522		$ 13,770	$ 13,578	$ 13,547
Net income attributable to Kellogg	$ 205	$ 348	$ 351	$ 347	$ 145	$ 247	$ 286	$ 282		1,251	960	1,336
Equity Method Investment, Realized Gain (Loss) on Disposal										$ 0	$ 0	200
Multipro
Business Acquisition [Line Items]
Net sales												536
Net income attributable to Kellogg												8
Equity Method Investment, Realized Gain (Loss) on Disposal									$ 245			$ 245