Retirement Plans (Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension settlement costs	$ 83
Special termination benefits	5	$ 26
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	97	88	$ 55
Interest cost on projected benefit obligation	127	122	88
Expected return on plan assets	(186)	(168)	(129)
Amortization of unrecognized loss	30	40	29
Net periodic benefit cost	68	82	43
Pension settlement costs	83
Special termination benefits	5	23
Total Disclosed Benefit Cost	156	105	43
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	21	19	19
Interest cost on projected benefit obligation	40	37	30
Amortization of prior service (credit)/costs	(9)	(9)	(5)
Amortization of unrecognized loss		1	8
Net periodic benefit cost	52	48	52
Special termination benefits		3
Total Disclosed Benefit Cost	$ 52	$ 51	$ 52