Quarterly Financial Data Net income and earnings per share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended									12 Months Ended
	Dec. 29, 2018		Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 30, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Net income (loss) attributable to Kellogg Company, Basic	$ (84)	[1]	$ 380	$ 596	$ 444	$ 417	$ 288	$ 283	$ 266	$ 1,336	$ 1,254	$ 699
Basic	$ (0.24)		$ 1.10	$ 1.72	$ 1.28	$ 1.21	$ 0.83	$ 0.81	$ 0.76	$ 3.85	$ 3.61	$ 1.99
Diluted	$ (0.24)		$ 1.09	$ 1.71	$ 1.27	$ 1.20	$ 0.83	$ 0.80	$ 0.75	$ 3.83	$ 3.58	$ 1.97

[1] The significant decrease in the fourth quarter 2018 net income is primarily due to a mark-to-market adjustment recognized on pension assets.