Equity Schedule of Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Equity [Abstract]
Net income (loss) attributable to Kellogg Company, Basic	$ (84)	[1]	$ 380	$ 596	$ 444	$ 417	$ 288	$ 283	$ 266	$ 1,336	$ 1,254	$ 699
Net income attributed to Kellogg Company, Diluted										$ 1,336	$ 1,254	$ 699
Average shares outstanding, Basic (in shares)										347	348	350
Average shares outstanding, Dilutive potential common shares (in shares)										1	2	4
Average shares outstanding, Diluted (in shares)										348	350	354
Earnings per share, basic	$ (0.24)		$ 1.10	$ 1.72	$ 1.28	$ 1.21	$ 0.83	$ 0.81	$ 0.76	$ 3.85	$ 3.61	$ 1.99
Earnings per share, Dilutive potential common shares (in dollars per share)										(0.02)	(0.03)	(0.02)
Earnings per share, 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.