Reportable Segments Supplemental product information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Segment Reporting Information [Line Items]
Net sales	$ 3,223	$ 3,372	$ 3,461	$ 3,522	$ 3,317	$ 3,469	$ 3,360	$ 3,401	$ 13,578	$ 13,547	$ 12,854
Snacks
Segment Reporting Information [Line Items]
Net sales									6,663	6,797	6,683
Retail Channel Cereal
Segment Reporting Information [Line Items]
Net sales									5,029	5,203	5,222
Frozen
Segment Reporting Information [Line Items]
Net sales									1,037	1,020	949
Noodles and other
Segment Reporting Information [Line Items]
Net sales									$ 849	$ 527	$ 0