Reportable Segments Reportable Segments Supplemental Product Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Segment Reporting Information [Line Items]
Net sales	$ 3,360	$ 3,175	$ 6,761	$ 6,423
Snacks
Segment Reporting Information [Line Items]
Net sales	1,684	1,635	3,458	3,351
Cereal
Segment Reporting Information [Line Items]
Net sales	1,304	1,305	2,655	2,603
Frozen and other
Segment Reporting Information [Line Items]
Net sales	$ 372	$ 235	$ 648	$ 469