Retirement Plans (Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Special termination benefits		$ 44
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 95	82	$ 90
Interest cost on projected benefit obligation	108	130	125
Expected return on plan assets	(171)	(172)	(192)
Amortization of unrecognized (gain) loss	99	58	24
Net periodic benefit cost	131	98	47
Pension settlement costs	159	57	41
Special termination benefits		38
Gain on disposal, net	(38)
Total benefit cost	252	193	88
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	20	20	21
Interest cost on projected benefit obligation	33	41	38
Amortization of prior service credit	(32)	(11)	(9)
Amortization of unrecognized (gain) loss	6	(4)	3
Net periodic benefit cost	27	46	53
Special termination benefits		6
Gain on disposal, net	(24)
Total benefit cost	$ 3	$ 52	$ 53