Reportable Segments Supplemental Product Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 28, 2020	Mar. 30, 2019
Segment Reporting Information [Line Items]
Net sales	$ 3,412	$ 3,522
Snacks
Segment Reporting Information [Line Items]
Net sales	1,325	1,780
Cereal
Segment Reporting Information [Line Items]
Net sales	1,554	1,275
Frozen
Segment Reporting Information [Line Items]
Net sales	294	271
Noodles and other
Segment Reporting Information [Line Items]
Net sales	$ 239	$ 196