Benefit Plans (Amounts Recorded in Accumulated Other Comprehensive Losses) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts recorded in accumulated other comprehensive losses	$ (1,839)	$ (2,048)
Net loss [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss	(3,198)	(3,537)
Prior service costs/credit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service (cost) credit	180	176
Deferred Income Taxes [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Deferred income taxes	1,179	1,313
Pension [Member] | Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts recorded in accumulated other comprehensive losses	(1,529)	(1,752)
Pension [Member] | Net loss [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss	(2,493)	(2,857)
Pension [Member] | Prior service costs/credit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service (cost) credit	(15)	(19)
Pension [Member] | Deferred Income Taxes [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Deferred income taxes	979	1,124
Postretirement Benefit Plan [Member] | Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts recorded in accumulated other comprehensive losses	(251)	(233)
Postretirement Benefit Plan [Member] | Net loss [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss	(612)	(581)
Postretirement Benefit Plan [Member] | Prior service costs/credit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service (cost) credit	195	195
Postretirement Benefit Plan [Member] | Deferred Income Taxes [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Deferred income taxes	166	153
Postemployment Retirement Benefits [Member] | Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts recorded in accumulated other comprehensive losses	(59)	(63)
Postemployment Retirement Benefits [Member] | Net loss [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss	(93)	(99)
Postemployment Retirement Benefits [Member] | Prior service costs/credit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prior service (cost) credit	0	0
Postemployment Retirement Benefits [Member] | Deferred Income Taxes [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Deferred income taxes	$ 34	$ 36