Pension and Other Postretirement Benefit Plans (Change in Plan Assets) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Other Postretirement Benefits
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year		$ 0	$ 0
Actual return on plan assets		0	0
Employer contribution		40	48
Plan participants' contributions		0	6
Benefits paid		(40)	(54)
Transfer of plan assets		0	0
Settlements		0	0
Foreign currency exchange rates		0	0
Fair value of plan assets, end of year	$ 0	0	0
United States Pension Benefits | Pension Benefits
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year		2,664	2,741
Actual return on plan assets		359	206
Employer contribution		47	14
Plan participants' contributions		0	0
Benefits paid		(260)	(286)
Transfer of plan assets	64	(64)	0
Settlements		0	(11)
Foreign currency exchange rates		0	0
Fair value of plan assets, end of year	2,746	2,746	2,664
Foreign Pension Benefits | Pension Benefits
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year		510	552
Actual return on plan assets		28	47
Employer contribution		30	30
Plan participants' contributions		1	1
Benefits paid		(32)	(31)
Transfer of plan assets		0	0
Settlements		(12)	(14)
Foreign currency exchange rates		46	(75)
Fair value of plan assets, end of year	$ 571	$ 571	$ 510