Reportable Segments Reportable Segments Supplemental Product Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Apr. 01, 2017
Segment Reporting Information [Line Items]
Net sales	$ 3,401	$ 3,248
Snacks
Segment Reporting Information [Line Items]
Net sales	1,775	1,717
Cereal
Segment Reporting Information [Line Items]
Net sales	1,351	1,298
Frozen
Segment Reporting Information [Line Items]
Net sales	$ 275	$ 233