Employee Retirement Plans (Net Periodic Benefit Expense) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 29	$ 26	$ 58	$ 51
Interest cost	31	37	62	74
Expected return on plan assets	(49)	(43)	(98)	(86)
Amortization of prior service cost (credit)	2	1	3	3
Recognition of actuarial (gain) loss	(2)	23	(2)	23
Total pension and postretirement benefit expense	11	44	23	65
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	2	5	4
Interest cost	5	7	10	14
Amortization of prior service cost (credit)	(2)	(2)	(3)	(4)
Total pension and postretirement benefit expense	$ 6	$ 7	$ 12	$ 14