Employee Retirement Plans (Net Periodic Benefit Expense) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 29	$ 25
Interest cost	31	37
Expected return on plan assets	(49)	(43)
Amortization of prior service cost (credit)	1	2
Total pension and postretirement benefit expense	12	21
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2
Interest cost	5	7
Amortization of prior service cost (credit)	(1)	(2)
Total pension and postretirement benefit expense	$ 6	$ 7