Reportable Segments Supplemental product information (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	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Segment Reporting Information [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
Snacks
Segment Reporting Information [Line Items]
Net sales									6,281	6,663	6,797
Retail Channel Cereal
Segment Reporting Information [Line Items]
Net sales									5,433	5,029	5,203
Frozen
Segment Reporting Information [Line Items]
Net sales									1,139	1,037	1,020
Noodles and other
Segment Reporting Information [Line Items]
Net sales									$ 917	$ 849	$ 527