Equity Changes in Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Equity [Abstract]
Net income	$ 977	$ 1,344	$ 1,254
Foreign currency translation adjustment before tax	100	5	(34)
Foreign currency translation adjustments tax (expense) benefit	(19)	(53)	113
Foreign currency translation adjustments after tax	81	(48)	79
Unrealized gain (loss) on cash flow hedges, pre-tax	5	3	0
Unrealized gain (loss) on cash flow hedges, tax (expense) benefit	(1)	(1)	0
Unrealized gain (loss) on cash flow hedges, after-tax	4	2	0
Reclassifications to net income, pre-tax	4	8	9
Reclassifications to net income, tax (expense) benefit	(1)	(2)	(3)
Reclassification to net income, after-tax	3	6	6
Net experience gain (loss)	(16)	(8)	44
Net experience gain (loss), tax (expense) benefit	5	1	(12)
Net experience gain (loss), after tax	(11)	(7)	32
Prior service credit (cost)	3	1	0
Prior service credit (cost), tax (expense) benefit	(1)	0	0
Prior service credit (cost), after-tax	2	1	0
Net experience (gain) loss, pre-tax	(5)	(5)	0
Net experience (gain) loss, tax (expense) benefit	1	1	0
Net experience loss, after-tax	(4)	(4)	0
Prior service (credit) cost, pre-tax	(1)	0	1
Prior service (credit) cost, tax (expense) benefit	0	0	0
Prior service cost, after-tax	(1)	0	1
Unrealized gain (loss) on available-for-sale securities, pre-tax	4	0	0
Unrealized gain (loss) on available-for-sale securities, tax (expense) benefit	0	0	0
Unrealized gain (loss) on available-for-sale securities, after-tax	4	0	0
Reclassification to net income on available-for-sale securities, pre-tax	(4)	0	0
Reclassification to net income on available-for-sale securities, tax (expense) benefit	0	0	0
Reclassification to net income on available-for-sale securities, after-tax	(4)	0	0
Other Comprehensive Income (Loss), before Tax	90	4	20
Other Comprehensive Income (Loss), Tax	(16)	(54)	98
Other Comprehensive Income (loss)	74	(50)	118
Comprehensive income	1,051	1,294	1,372
Net income (loss) attributable to noncontrolling interests	17	8	0
Other comprehensive income (loss) attributable to noncontrolling interests	0	(7)	0
Comprehensive income attributable to Kellogg Company	$ 1,034	$ 1,293	$ 1,372