Reportable Segments Supplemental Product Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 28, 2019	Sep. 29, 2018
Segment Reporting Information [Line Items]
Net sales	$ 3,372	$ 3,469	$ 10,355	$ 10,230
Snacks
Segment Reporting Information [Line Items]
Net sales	1,610	1,688	5,132	5,146
Cereal
Segment Reporting Information [Line Items]
Net sales	1,280	1,330	3,811	3,985
Frozen
Segment Reporting Information [Line Items]
Net sales	260	258	786	781
Noodles and other
Segment Reporting Information [Line Items]
Net sales	$ 222	$ 193	$ 626	$ 318