Consolidated Statement of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 28, 2020	Mar. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 350	$ 285
Other comprehensive income (loss), pre-tax:
Foreign currency translation adjustments, pre-tax	(241)	66
Cash flow hedges, pre-tax:
Unrealized gain (loss) on cash flow hedges, pre-tax	(65)
Reclassification to net income, pre-tax	2	1
Postretirement and postemployment benefits reclassification to net income, pre-tax:
Net experience (gain) loss	(1)	(1)
Available-for-sale securities, pre-tax
Unrealized gain (loss) on available-for-sale securities, pre-tax	(3)	2
Other comprehensive income (loss), pre-tax	(308)	68
Other comprehensive income (loss), tax (expense) benefit
Foreign currency translation adjustments, tax (expense) benefit	(20)	(10)
Cash flow hedges, tax (expense) benefit:
Unrealized gain (loss) on cash flow hedges, tax (expense) benefit	17
Reclassification to net income, tax (expense) benefit	0	0
Postretirement and postemployment benefits reclassification to net income, tax (expense) benefit:
Net experience loss, 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
Other comprehensive income (loss), tax (expense) benefit	(3)	(10)
Other comprehensive income (loss), after tax:
Foreign currency translation adjustments, after-tax	(261)	56
Cash flow hedges, after tax
Unrealized gain (loss) on cash flow hedges, after-tax	(48)
Reclassification to net income, after-tax	2	1
Postretirement and postemployment benefits reclassification to net income, after-tax:
Net experience (gain) loss, after-tax	(1)	(1)
Available-for-sale securities, after tax
Unrealized gain (loss) on available-for-sale securities, after-tax	(3)	2
Other comprehensive income (loss)	(311)	58
Comprehensive income	39	343
Net income attributable to noncontrolling interests	3	3
Other comprehensive income (loss) attributable to noncontrolling interests	(32)	3
Comprehensive income attributable to Kellogg Company	$ 68	$ 337