Benefit Plans (Amounts Recorded in Accumulated Other Comprehensive Losses) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Net losses	$ (4,100)	$ (3,845)	$ (3,558)
Prior service cost	(49)	(35)	(39)
Net transition obligation	(5)	(5)	(6)
Deferred income taxes	589	553	583
Losses to be amortized	(3,565)	(3,332)	(3,020)
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Net losses	(3,314)	(3,074)	(2,760)
Prior service cost	(53)	(40)	(45)
Net transition obligation	(5)	(5)	(6)
Deferred income taxes	350	320	342
Losses to be amortized	(3,022)	(2,799)	(2,469)
Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Net losses	(73)	(61)	(77)
Prior service cost	4	5	6
Net transition obligation	0	0	0
Deferred income taxes	24	20	25
Losses to be amortized	(45)	(36)	(46)
Postemployment [Member]
Defined Benefit Plan Disclosure [Line Items]
Net losses	(713)	(710)	(721)
Prior service cost	0	0	0
Net transition obligation	0	0	0
Deferred income taxes	215	213	216
Losses to be amortized	$ (498)	$ (497)	$ (505)