Retirement Plans (Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension settlement costs	$ 15		$ 77
Pension Costs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	26	$ 16	76	$ 63
Interest cost on projected benefit obligation	27	24	94	89
Expected return on plan assets	(43)	(30)	(139)	(122)
Amortization of unrecognized (gain) loss	6	9	23	30
Net periodic pension benefit cost, before settlement	16	19	54	60
Pension settlement costs	15		77
Net periodic benefit cost	31	19	131	60
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5	6	16	14
Interest cost on projected benefit obligation	11	10	30	27
Amortization of prior service cost/(credit)	(2)	(2)	(7)	(7)
Amortization of unrecognized (gain) loss	(1)			1
Net periodic benefit cost	$ 13	$ 14	$ 39	$ 35