Consolidated Statement of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 27, 2020	Jun. 29, 2019	Jun. 27, 2020	Jun. 29, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 354	$ 292	$ 704	$ 577
Other comprehensive income (loss), pre-tax:
Foreign currency translation adjustments, pre-tax	(20)	(5)	(261)	61
Cash flow hedges, pre-tax:
Unrealized gain (loss) on cash flow hedges, pre-tax	12		(53)
Reclassification to net income, pre-tax	5	2	7	3
Postretirement and postemployment benefits reclassification to net income, pre-tax:
Net experience (gain) loss	(1)	(1)	(2)	(2)
Prior service cost	(1)		(1)
Available-for-sale securities, pre-tax
Unrealized gain (loss) on available-for-sale securities, pre-tax	5	1	2	3
Other comprehensive income (loss), pre-tax	0	(3)	(308)	65
Other comprehensive income (loss), tax (expense) benefit
Foreign currency translation adjustments, tax (expense) benefit	10	1	(10)	(9)
Cash flow hedges, tax (expense) benefit:
Unrealized gain (loss) on cash flow hedges, tax (expense) benefit	(3)		14
Reclassification to net income, tax (expense) benefit	(2)	(1)	(2)	(1)
Postretirement and postemployment benefits reclassification to net income, tax (expense) benefit:
Net experience loss, tax (expense) benefit	1	1	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
Other comprehensive income (loss), tax (expense) benefit	6	1	3	(9)
Other comprehensive income (loss), after tax:
Foreign currency translation adjustments, after-tax	(10)	(4)	(271)	52
Cash flow hedges, after tax
Unrealized gain (loss) on cash flow hedges, after-tax	9		(39)
Reclassification to net income, after-tax	3	1	5	2
Postretirement and postemployment benefits reclassification to net income, after-tax:
Net experience (gain) loss, after-tax	0	0	(1)	(1)
Prior service cost, after-tax	(1)		(1)
Available-for-sale securities, after tax
Unrealized gain (loss) on available-for-sale securities, after-tax	5	1	2	3
Other comprehensive income (loss)	6	(2)	(305)	56
Comprehensive income	360	290	399	633
Net income attributable to noncontrolling interests	3	6	6	9
Other comprehensive income (loss) attributable to noncontrolling interests	(4)	0	(36)	3
Comprehensive income attributable to Kellogg Company	$ 361	$ 284	$ 429	$ 621