Employee Retirement Plans (Net Periodic Benefit Expense) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plan [Member]
Service cost	$ 23	$ 21	$ 47	$ 43
Interest cost	31	31	62	62
Expected return on plan assets	(44)	(41)	(87)	(83)
Amortization of prior service cost (credit)	2	2	3	3
Recognition of actuarial loss	15	28	15	35
Total pension and postretirement benefit expense	27	41	40	60
Other Postretirement Benefit Plan [Member]
Service cost	2	2	5	4
Interest cost	6	6	13	13
Amortization of prior service cost (credit)		(1)	(1)	(2)
Total pension and postretirement benefit expense	$ 8	$ 7	$ 17	$ 15