Benefit Plans (Projected Benefit Obligations, Plan Assets and Funded Status of Pension Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets:
Accrued pension costs	$ (551)	$ (473)
Accrued postretirement health care costs	(1,951)	(1,797)
Pension [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	8,659	7,726
Service cost	74	70	$ 81
Interest cost	251	306	276
Benefits paid	(477)	(493)
Actuarial losses	970	1,025
Settlement and curtailment	0	25
Other	(12)	0
Benefit obligation at end of year	9,465	8,659	7,726
Change in plan assets:
Fair value of plan assets at beginning of year	8,167	7,138
Actual return on plan assets	1,188	1,466
Employer contributions	33	56
Benefits paid	(477)	(493)
Fair value of plan assets at end of year	8,911	8,167	7,138
Funded status at December 31	(554)	(492)
Other accrued liabilities	(23)	(26)
Accrued pension costs	(551)	(473)
Other assets	20	7
Accrued postretirement health care costs	0	0
Defined benefit plan, amounts for asset (liability) recognized in statement of financial position	(554)	(492)
Postretirement Benefit Plan [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	2,091	2,040
Service cost	16	16	18
Interest cost	59	76	70
Benefits paid	(107)	(126)
Actuarial losses	169	78
Settlement and curtailment	0	7
Other	1	0
Benefit obligation at end of year	2,229	2,091	2,040
Change in plan assets:
Fair value of plan assets at beginning of year	213	211
Actual return on plan assets	21	45
Employer contributions	0	0
Benefits paid	(33)	(43)
Fair value of plan assets at end of year	201	213	$ 211
Funded status at December 31	(2,028)	(1,878)
Other accrued liabilities	(77)	(81)
Accrued pension costs	0	0
Other assets	0	0
Accrued postretirement health care costs	(1,951)	(1,797)
Defined benefit plan, amounts for asset (liability) recognized in statement of financial position	$ (2,028)	$ (1,878)