Retirement Plans (Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 21	$ 24
Interest cost on projected benefit obligation	33	30
Expected return on plan assets	(43)	(50)
Amortization of unrecognized (gain) loss	14	7
Net periodic benefit cost	25	11
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5	5
Interest cost on projected benefit obligation	10	9
Amortization of prior service cost (credit)	(1)	(2)
Amortization of unrecognized (gain) loss	(2)	1
Net periodic benefit cost	$ 12	$ 13