Accumulated Other Comprehensive Loss - Schedule of AOCI Components (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax
Beginning balance	$ 12,287	$ 10,850	$ 8,813
Change in value (net of tax effect)	(137)	(459)	8
Reclassification into earnings (net of tax effect)	152	98	28
Ending balance	13,910	12,287	10,850
AOCI beginning balance, tax effect	(1,458)	(1,222)	(1,279)
Changes in value, tax effect	(32)	(293)	41
Reclassifications into earnings, tax effect	90	57	16
AOCI ending balance, tax effect	(1,400)	(1,458)	(1,222)
Deferred income taxes, pension obligation	700
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax
Beginning balance	(7,636)	(7,275)	(7,311)
Ending balance	(7,621)	(7,636)	(7,275)
Pension and Other Benefits Liabilities
AOCI Attributable to Parent, Net of Tax
Beginning balance	(7,714)	(7,354)	(7,517)
Change in value (net of tax effect)	(264)	(482)	10
Reclassification into earnings (net of tax effect)	166	122	153
Ending balance	(7,812)	(7,714)	(7,354)
Deferred income taxes, pension obligation	700
Derivative Contracts
AOCI Attributable to Parent, Net of Tax
Beginning balance	97	140	222
Change in value (net of tax effect)	(21)	(19)	43
Reclassification into earnings (net of tax effect)	(6)	(24)	(125)
Ending balance	70	97	140
Investments
AOCI Attributable to Parent, Net of Tax
Beginning balance	(19)	(61)	(16)
Change in value (net of tax effect)	148	42	(45)
Reclassification into earnings (net of tax effect)	(8)	0	0
Ending balance	$ 121	$ (19)	$ (61)