Employee Retirement Plans (Net Periodic Benefit Expense) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Service cost	$ 25	$ 24
Interest cost	32	31
Expected return on plan assets	(47)	(43)
Amortization of prior service cost (credit)	2	1
Total net periodic benefit cost	12	13
Postretirement Benefits [Member]
Service cost	3	3
Interest cost	6	7
Amortization of prior service cost (credit)	(1)	(1)
Total net periodic benefit cost	$ 8	$ 9