Retirement Plans (Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total benefit cost	$ 20	$ 125
Pension Costs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	24	25
Interest cost on projected benefit obligation	23	30
Expected return on plan assets	(45)	(50)
Amortization of unrecognized (gain) loss	18	17
Net periodic benefit cost	20	22
Pension settlement costs		103
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	7	8
Amortization of prior service credit	(7)	(8)
Amortization of unrecognized (gain) loss	3	2
Net periodic benefit cost	$ 8	$ 7