Accounting Policies Impacts of Adoption of Standards Related to Revenue Recognition, Leases and Cash Flow to Previously Reported Results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017	Dec. 30, 2017
Consolidated Balance Sheet
Other assets	$ 1,223		$ 1,223		$ 1,027
Other current liabilities	1,455		1,455		1,474
Deferred income taxes	752		752		355
Retained earnings	7,929		7,929		7,069
Consolidated Statement of Income
Net sales	3,469	$ 3,246	10,230	$ 9,669
Cost of goods sold	2,293	2,074	6,593	6,112
Selling, general and administrative expense	780	839	2,257	2,559
Other income (expense), net	130	117	269	268
Income taxes	69	101	206	246
Net income attributable to Kellogg Company	$ 380	$ 288	$ 1,420	$ 837
Per share amounts:
Basic earnings (in dollars per share)	$ 1.10	$ 0.83	$ 4.10	$ 2.40
Diluted earnings (in dollars per share)	$ 1.09	$ 0.83	$ 4.07	$ 2.38
Consolidated Statement of Cash Flows
Net income	$ 383	$ 288	$ 1,426	$ 837	1,254
Deferred income taxes			99	(21)
Other			(91)	46
Trade receivables			(139)	(1,168)
All other current assets and liabilities			(154)	88
Net cash provided by (used in) operating activities			926	190
Collections of deferred purchase price on securitized trade receivables			0	945
Investment in unconsolidated entities, net proceeds				0
Net cash provided by (used in) investing activities			$ (785)	568
As previously reported
Consolidated Balance Sheet
Other assets					1,026
Other current liabilities					1,431
Deferred income taxes					363
Retained earnings					7,103
Consolidated Statement of Income
Net sales		3,273		9,714
Cost of goods sold		2,041		6,013
Selling, general and administrative expense		768		2,424
Other income (expense), net		(2)		(5)
Income taxes		104		248
Net income attributable to Kellogg Company		$ 297		$ 841
Per share amounts:
Basic earnings (in dollars per share)		$ 0.86		$ 2.41
Diluted earnings (in dollars per share)		$ 0.85		$ 2.39
Consolidated Statement of Cash Flows
Net income				$ 841
Deferred income taxes				(20)
Other				32
Trade receivables				(223)
All other current assets and liabilities				83
Net cash provided by (used in) operating activities				1,121
Collections of deferred purchase price on securitized trade receivables				0
Investment in unconsolidated entities, net proceeds				14
Net cash provided by (used in) investing activities				(363)
Revenue recognition ASU
Consolidated Balance Sheet
Other assets					1
Other current liabilities					43
Deferred income taxes					(8)
Retained earnings					$ (34)
Consolidated Statement of Income
Net sales		$ (27)		(45)
Cost of goods sold		(21)		(54)
Selling, general and administrative expense		6		15
Other income (expense), net		0		0
Income taxes		(3)		(2)
Net income attributable to Kellogg Company		$ (9)		$ (4)
Per share amounts:
Basic earnings (in dollars per share)		$ (0.03)		$ (0.01)
Diluted earnings (in dollars per share)		$ (0.02)		$ (0.01)
Consolidated Statement of Cash Flows
Net income				$ (4)
Deferred income taxes				(1)
Other				0
Trade receivables				0
All other current assets and liabilities				5
Net cash provided by (used in) operating activities				0
Collections of deferred purchase price on securitized trade receivables				0
Investment in unconsolidated entities, net proceeds				0
Net cash provided by (used in) investing activities				0
Pension ASU
Consolidated Statement of Income
Net sales		$ 0		0
Cost of goods sold		54		153
Selling, general and administrative expense		65		120
Other income (expense), net		119		273
Income taxes		0		0
Net income attributable to Kellogg Company		$ 0		$ 0
Per share amounts:
Basic earnings (in dollars per share)		$ 0.00		$ 0.00
Diluted earnings (in dollars per share)		$ 0.00		$ 0.00
Cash Flow ASU
Consolidated Statement of Cash Flows
Net income				$ 0
Deferred income taxes				0
Other				14
Trade receivables				(945)
All other current assets and liabilities				0
Net cash provided by (used in) operating activities				(931)
Collections of deferred purchase price on securitized trade receivables				945
Investment in unconsolidated entities, net proceeds				(14)
Net cash provided by (used in) investing activities				$ 931