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, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in plan assets:
Employer contributions	$ 270
Accrued pension costs	(445)	$ (445)	$ (805)
Accrued postretirement health care costs	(1,987)	(1,987)	(2,217)
Pension [Member]
Change in benefit obligation:
Benefit obligation at beginning of year		8,312	8,011
Service cost		75	76	$ 86
Interest cost		288	281	337
Benefits paid		(703)	(440)
Actuarial losses		589	367
Termination, settlement and curtailment		(51)	13
Other		0	4
Benefit obligation at end of year	8,510	8,510	8,312	8,011
Change in plan assets:
Fair value of plan assets at beginning of year		7,475	6,706
Actual return on plan assets		1,219	678
Employer contributions		24	531
Benefits paid		(703)	(440)
Fair value of plan assets at end of year	8,015	8,015	7,475	6,706
Funded status at December 31	(495)	(495)	(837)
Other accrued liabilities	(51)	(51)	(32)
Accrued pension costs	(445)	(445)	(805)
Other assets	1	1
Accrued postretirement health care costs	0	0	0
Postretirement Benefit Plan [Member]
Change in benefit obligation:
Benefit obligation at beginning of year		2,364	2,392
Service cost		16	17	18
Interest cost		76	77	100
Benefits paid		(139)	(135)
Actuarial losses		56	24
Termination, settlement and curtailment		0	5
Other		(38)	(16)
Benefit obligation at end of year	2,335	2,335	2,364	2,392
Change in plan assets:
Fair value of plan assets at beginning of year		0	0
Actual return on plan assets		0	0
Employer contributions		270	0
Benefits paid		0	0
Fair value of plan assets at end of year	270	270	0	$ 0
Funded status at December 31	(2,065)	(2,065)	(2,364)
Other accrued liabilities	(78)	(78)	(147)
Accrued pension costs	0	0	0
Accrued postretirement health care costs	$ (1,987)	$ (1,987)	$ (2,217)