Retirement Plans - Net periodic benefit cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Net periodic benefit cost
Other components of net periodic benefit cost (credit)	$ (19)	$ (7)	$ (57)	$ (21)
Cost of sales	2,687	2,884	8,489	8,472
Pension Benefits
Net periodic benefit cost
Service cost	26	25	79	75
Interest cost	77	81	230	242
Expected return on plan assets	(138)	(127)	(415)	(380)
Amortization of net actuarial loss	38	34	115	102
Amortization of prior service cost (credit)	4	4	11	12
Net periodic benefit cost	7	17	20	51
Postretirement Benefits Other Than Pensions
Net periodic benefit cost
Service cost	1	1	2	2
Interest cost	2	3	7	9
Amortization of prior service cost (credit)	(2)	(2)	(5)	(6)
Net periodic benefit cost	$ 1	2	$ 4	5
Accounting Standards Update 2017-07
Net periodic benefit cost
Other components of net periodic benefit cost (credit)		(7)		(21)
Cost of sales		$ 7		$ 21