Employee Retirement Plans (Net Periodic Benefit Expense) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits [Member]
Service cost	$ 26	$ 22	$ 77	$ 69
Interest cost	32	32	97	94
Expected return on plan assets	(47)	(43)	(142)	(130)
Amortization of prior service cost (credit)	2	1	6	4
Recognition of actuarial loss			1	15
Total pension and postretirement benefit expense	13	12	39	52
Postretirement Benefits [Member]
Service cost	3	2	9	7
Interest cost	6	7	18	20
Amortization of prior service cost (credit)	(2)	(1)	(5)	(2)
Total pension and postretirement benefit expense	$ 7	$ 8	$ 22	$ 25