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	577			568			568	9
Sale of subsidiary shares to noncontrolling interest	1						0	1
Dividends declared	(380)			(380)			(380)
Distributions to noncontrolling interest	(9)						0	(9)
Other comprehensive income (loss)	56					53	53	3
Reclassification of tax effects relating to U.S. tax reform	0			22		(22)	0
Stock compensation	29		29				29
Stock options exercised and other (in shares)					(1)
Stock options exercised and other	(1)		(29)	(4)	$ 32		(1)
Balance (in shares) at Jun. 29, 2019		421			80
Balance at Jun. 29, 2019		$ 105	895	7,858	$ (4,739)	(1,469)	2,650	562
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,097
Balance at Mar. 30, 2019		$ 105	877	7,762	$ (4,744)	(1,467)	2,533	564
Balance (in shares) at Mar. 30, 2019		421			80
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	292			286			286	6
Sale of subsidiary shares to noncontrolling interest	1						0	1
Dividends declared	(188)			(188)			(188)
Distributions to noncontrolling interest	(9)						0	(9)
Other comprehensive income (loss)	(2)					(2)	(2)	0
Reclassification of tax effects relating to U.S. tax reform	0			0		0	0
Stock compensation	16		16				16
Stock options exercised and other (in shares)					0
Stock options exercised and other	5		2	(2)	$ 5		5
Balance (in shares) at Jun. 29, 2019		421			80
Balance at Jun. 29, 2019		$ 105	895	7,858	$ (4,739)	(1,469)	2,650	562
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,212
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	704			698			698	6
Dividends declared	(390)			(390)			(390)
Distributions to noncontrolling interest	(1)						0	(1)
Other comprehensive income (loss)	(305)					(269)	(269)	(36)
Stock compensation	37		37				37
Stock options exercised and other (in shares)					(1)
Stock options exercised and other	47		(29)	(1)	$ 77		47
Balance (in shares) at Jun. 27, 2020		421			78
Balance at Jun. 27, 2020	2,870	$ 105	929	8,166	$ (4,613)	(1,717)	2,870	536
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,212
Balance at Mar. 28, 2020		$ 105	911	8,010	$ (4,625)	(1,727)	2,674	538
Balance (in shares) at Mar. 28, 2020		421			78
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	354			351			351	3
Dividends declared	(195)			(195)			(195)
Distributions to noncontrolling interest	(1)						0	(1)
Other comprehensive income (loss)	6					10	10	(4)
Stock compensation	18		18				18
Stock options exercised and other (in shares)					0
Stock options exercised and other	12		0	0	$ 12		12
Balance (in shares) at Jun. 27, 2020		421			78
Balance at Jun. 27, 2020	2,870	$ 105	$ 929	$ 8,166	$ (4,613)	$ (1,717)	$ 2,870	$ 536
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 3,406