Benefit Plans (Projected Benefit Obligations, Plan Assets and Funded Status of Pension Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in plan assets:
Accrued pension costs	$ (473)	$ (544)
Accrued postretirement health care costs	(1,797)	(1,749)
Pension [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	7,726	8,510
Service cost	70	81	$ 75
Interest cost	306	276	288
Benefits paid	(493)	(488)
Actuarial (gains) losses	1,025	(660)
Settlement and curtailment	25	(18)
Other	0	25
Benefit obligation at end of year	8,659	7,726	8,510
Change in plan assets:
Fair value of plan assets at beginning of year	7,138	8,015
Actual return on plan assets	1,466	(430)
Employer contributions	56	41
Benefits paid	(493)	(488)
Fair value of plan assets at end of year	8,167	7,138	8,015
Funded status at December 31	(492)	(588)
Other accrued liabilities	(26)	(44)
Accrued pension costs	(473)	(544)
Other assets	7	0
Accrued postretirement health care costs	0	0
Defined benefit plan, amounts for asset (liability) recognized in statement of financial position	(492)	(588)
Postretirement Benefit Plan [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	2,040	2,335
Service cost	16	18	16
Interest cost	76	70	76
Benefits paid	(126)	(130)
Actuarial (gains) losses	78	(298)
Settlement and curtailment	7	0
Other	0	45
Benefit obligation at end of year	2,091	2,040	2,335
Change in plan assets:
Fair value of plan assets at beginning of year	211	270
Actual return on plan assets	45	(14)
Employer contributions	0	0
Benefits paid	(43)	(45)
Fair value of plan assets at end of year	213	211	$ 270
Funded status at December 31	(1,878)	(1,829)
Other accrued liabilities	(81)	(80)
Accrued pension costs	0	0
Other assets	0	0
Accrued postretirement health care costs	(1,797)	(1,749)
Defined benefit plan, amounts for asset (liability) recognized in statement of financial position	$ (1,878)	$ (1,829)