Retirement Plans - Net periodic benefit cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Net periodic benefit cost
Other components of net periodic benefit cost (credit)	$ (19)	$ (6)	$ (38)	$ (14)
Cost of sales	3,073	2,996	5,802	5,588
Pension Benefits
Net periodic benefit cost
Service cost	27	25	53	50
Interest cost	76	81	153	161
Expected return on plan assets	(139)	(127)	(277)	(253)
Amortization of prior service cost (credit)	39	34	77	68
Amortization of net actuarial loss	3	4	7	8
Net periodic benefit cost	6	17	13	34
Postretirement Benefits Other Than Pensions
Net periodic benefit cost
Service cost			1	1
Interest cost	3	3	5	6
Amortization of prior service cost (credit)	(1)	(2)	(3)	(4)
Net periodic benefit cost	$ 2	1	$ 3	3
Accounting Standards Update 2017-07
Net periodic benefit cost
Other components of net periodic benefit cost (credit)		(6)		(14)
Cost of sales		$ 6		$ 14