Benefit Plans (Projected Benefit Obligations, Plan Assets and Funded Status of Pension Plans) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in plan assets:
Fair value of plan assets at beginning of year		$ 6,706
Fair value of plan assets at end of year		7,475	$ 6,706
Accrued pension costs		(805)	(1,277)
Accrued postretirement health care costs		(2,217)	(2,245)
Pension [Member]
Change in benefit obligation:
Benefit obligation at beginning of year		8,011	8,330
Service cost		76	86	$ 68
Interest cost		281	337	345
Benefits paid		(440)	(431)
Actuarial losses (gains)		367	(317)
Termination and curtailment		13	0
Other		4	6
Benefit obligation at end of year		8,312	8,011	8,330
Change in plan assets:
Fair value of plan assets at beginning of year		6,706	7,297
Actual return on plan assets		678	(188)
Employer contributions	$ 500	531	28
Benefits paid		(440)	(431)
Fair value of plan assets at end of year		7,475	6,706	7,297
Funded status at December 31		(837)	(1,305)
Other accrued liabilities		(32)	(28)
Accrued pension costs		(805)	(1,277)
Accrued postretirement health care costs		0	0
Postretirement Benefit Plan [Member]
Change in benefit obligation:
Benefit obligation at beginning of year		2,392	2,613
Service cost		17	18	15
Interest cost		77	100	107
Benefits paid		(135)	(141)
Actuarial losses (gains)		24	(192)
Termination and curtailment		5	0
Other		(16)	(6)
Benefit obligation at end of year		2,364	2,392	$ 2,613
Change in plan assets:
Benefits paid		(135)	(141)
Funded status at December 31		(2,364)	(2,392)
Other accrued liabilities		(147)	(147)
Accrued pension costs		0	0
Accrued postretirement health care costs		$ (2,217)	$ (2,245)