Benefit Plans (Projected Benefit Obligations, Plan Assets and Funded Status of Pension Plans) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets:
Employer contributions	$ 270
Accrued pension costs	(445)	$ (544)	$ (445)
Accrued postretirement health care costs	(1,987)	(1,749)	(1,987)
Pension [Member]
Change in benefit obligation:
Benefit obligation at beginning of year		8,510	8,312
Service cost		81	75	$ 76
Interest cost		276	288	281
Benefits paid		(488)	(703)
Actuarial (gains) losses		(660)	589
Termination, settlement and curtailment		(18)	(51)
Other		25	0
Benefit obligation at end of year	8,510	7,726	8,510	8,312
Change in plan assets:
Fair value of plan assets at beginning of year		8,015	7,475
Actual return on plan assets		(430)	1,219
Employer contributions		41	24
Benefits paid		(488)	(703)
Fair value of plan assets at end of year	8,015	7,138	8,015	7,475
Funded status at December 31	(495)	(588)	(495)
Other accrued liabilities	(51)	(44)	(51)
Accrued pension costs	(445)	(544)	(445)
Other assets	1	0	1
Accrued postretirement health care costs	0	0	0
Defined benefit plan, amounts for asset (liability) recognized in statement of financial position	(495)	(588)	(495)
Postretirement Benefit Plan [Member]
Change in benefit obligation:
Benefit obligation at beginning of year		2,335	2,364
Service cost		18	16	17
Interest cost		70	76	77
Benefits paid		(130)	(139)
Actuarial (gains) losses		(298)	56
Termination, settlement and curtailment		0	0
Other		45	(38)
Benefit obligation at end of year	2,335	2,040	2,335	2,364
Change in plan assets:
Fair value of plan assets at beginning of year		270	0
Actual return on plan assets		(14)	0
Employer contributions		0	270
Benefits paid		(45)	0
Fair value of plan assets at end of year	270	211	270	$ 0
Funded status at December 31	(2,065)	(1,829)	(2,065)
Other accrued liabilities	(78)	(80)	(78)
Accrued pension costs	0	0	0
Other assets	0	0	0
Accrued postretirement health care costs	(1,987)	(1,749)	(1,987)
Defined benefit plan, amounts for asset (liability) recognized in statement of financial position	$ (2,065)	$ (1,829)	$ (2,065)