Retirement Plans - Net periodic benefit cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 30, 2019	Mar. 31, 2018
Pension Benefits
Net periodic benefit cost
Service cost	$ 23	$ 26
Interest cost	82	77
Expected return on plan assets	(139)	(138)
Amortization of prior service cost (credit)	3	4
Amortization of net actuarial loss	25	38
Net periodic benefit cost (credit)	(6)	7
Postretirement Benefits Other Than Pensions
Net periodic benefit cost
Service cost	1	1
Interest cost	2	2
Amortization of prior service cost (credit)	(2)	(2)
Net periodic benefit cost (credit)	$ 1	$ 1