Reportable Segments Reportable Segments Supplemental Product Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net sales	$ 3,469	$ 3,246	$ 10,230	$ 9,669
Snacks
Segment Reporting Information [Line Items]
Net sales	1,688	1,649	5,146	5,000
Cereal
Segment Reporting Information [Line Items]
Net sales	1,330	1,355	3,985	3,958
Frozen and other
Segment Reporting Information [Line Items]
Net sales	$ 451	$ 242	$ 1,099	$ 711