Consolidated Statement of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 352	$ 248	$ 1,056	$ 825
Other comprehensive income (loss), pre-tax:
Foreign currency translation adjustments, pre-tax	(91)	35	(352)	96
Cash flow hedges, pre-tax:
Unrealized gain (loss) on cash flow hedges, pre-tax	15	(12)	(38)	(12)
Reclassification to net income, pre-tax	4	(1)	11	2
Postretirement and postemployment benefits reclassification to net income, pre-tax:
Net experience (gain) loss	0	(1)	(2)	(3)
Prior service cost	0		(1)
Available-for-sale securities, pre-tax
Unrealized gain (loss) on available-for-sale securities, pre-tax	0	1	2	4
Reclassification to net income on available-for-sale securities, pre-tax		(4)		(4)
Other comprehensive income (loss), pre-tax	(72)	18	(380)	83
Other comprehensive income (loss), tax (expense) benefit
Foreign currency translation adjustments, tax (expense) benefit	39	(38)	29	(47)
Cash flow hedges, tax (expense) benefit:
Unrealized gain (loss) on cash flow hedges, tax (expense) benefit	(4)	4	10	4
Reclassification to net income, tax (expense) benefit	(1)	0	(3)	(1)
Postretirement and postemployment benefits reclassification to net income, tax (expense) benefit:
Net experience loss, tax (expense) benefit	0	0	1	1
Prior service cost, tax (expense) benefit	0		0
Available-for-sale securities, tax (expense) benefit
Unrealized gain (loss) on available-for-sale securities, tax (expense) benefit	0	0	0	0
Reclassification to net income on available for sale of securities, tax (expense) benefit		0		0
Other comprehensive income (loss), tax (expense) benefit	34	(34)	37	(43)
Other comprehensive income (loss), after tax:
Foreign currency translation adjustments, after-tax	(52)	(3)	(323)	49
Cash flow hedges, after tax
Unrealized gain (loss) on cash flow hedges, after-tax	11	(8)	(28)	(8)
Reclassification to net income, after-tax	3	(1)	8	1
Postretirement and postemployment benefits reclassification to net income, after-tax:
Net experience (gain) loss, after-tax	0	(1)	(1)	(2)
Prior service cost, after-tax	0		(1)
Available-for-sale securities, after tax
Unrealized gain (loss) on available-for-sale securities, after-tax	0	1	2	4
Reclassification to net income on available for sale securities, after tax		(4)		(4)
Other comprehensive income (loss)	(38)	(16)	(343)	40
Comprehensive income	314	232	713	865
Net income attributable to noncontrolling interests	4	1	10	10
Other comprehensive income (loss) attributable to noncontrolling interests	2	(2)	(34)	1
Comprehensive income attributable to Kellogg Company	$ 308	$ 233	$ 737	$ 854