Consolidated Statement of Equity (Unaudited) - USD ($) shares in Millions, $ in Millions	Total	Common stock	Capital in excess of par value	Retained earnings	Treasury stock	Accumulated other comprehensive income (loss)	Total Kellogg Company equity	Non-controlling interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 3,159
Balance at Dec. 29, 2018		$ 105	$ 895	$ 7,652	$ (4,551)	$ (1,500)	$ 2,601	$ 558
Balance (in shares) at Dec. 29, 2018		421			77
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock repurchases (in shares)					4
Common stock repurchases	(220)				$ (220)		(220)
Net income	825			815			815	10
Sale of subsidiary shares to noncontrolling interest	1						0	1
Dividends declared	(574)			(574)			(574)
Distributions to noncontrolling interest	(9)						0	(9)
Other comprehensive income (loss)	40					39	39	1
Reclassification of tax effects relating to U.S. tax reform	0			22		(22)	0
Stock compensation	42		42				42
Stock options exercised and other (in shares)					(1)
Stock options exercised and other	24		(28)	(5)	$ 57		24
Balance (in shares) at Sep. 28, 2019		421			80
Balance at Sep. 28, 2019		$ 105	909	7,910	$ (4,714)	(1,483)	2,727	561
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,212
Balance at Jun. 29, 2019		$ 105	895	7,858	$ (4,739)	(1,469)	2,650	562
Balance (in shares) at Jun. 29, 2019		421			80
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	248			247			247	1
Dividends declared	(194)			(194)			(194)
Other comprehensive income (loss)	(16)					(14)	(14)	(2)
Stock compensation	13		13				13
Stock options exercised and other (in shares)					0
Stock options exercised and other	25		1	(1)	$ 25		25
Balance (in shares) at Sep. 28, 2019		421			80
Balance at Sep. 28, 2019		$ 105	909	7,910	$ (4,714)	(1,483)	2,727	561
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,288
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,314
Balance at Dec. 28, 2019	2,747	$ 105	921	7,859	$ (4,690)	(1,448)	2,747	567
Balance (in shares) at Dec. 28, 2019		421			79
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,056			1,046			1,046	10
Dividends declared	(586)			(586)			(586)
Distributions to noncontrolling interest	(1)						0	(1)
Other comprehensive income (loss)	(343)					(309)	(309)	(34)
Stock compensation	55		55				55
Stock options exercised and other (in shares)					(2)
Stock options exercised and other	91		(28)	(1)	$ 120		91
Balance (in shares) at Sep. 26, 2020		421			77
Balance at Sep. 26, 2020	3,044	$ 105	948	8,318	$ (4,570)	(1,757)	3,044	542
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,406
Balance at Jun. 27, 2020		$ 105	929	8,166	$ (4,613)	(1,717)	2,870	536
Balance (in shares) at Jun. 27, 2020		421			78
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	352			348			348	4
Dividends declared	(196)			(196)			(196)
Other comprehensive income (loss)	(38)					(40)	(40)	2
Stock compensation	18		18				18
Stock options exercised and other (in shares)					(1)
Stock options exercised and other	44		1	0	$ 43		44
Balance (in shares) at Sep. 26, 2020		421			77
Balance at Sep. 26, 2020	3,044	$ 105	$ 948	$ 8,318	$ (4,570)	$ (1,757)	$ 3,044	$ 542
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 3,586