Accumulated Other Comprehensive Income - Components (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	$ (3,241)	$ (3,400)
Cumulative effect of change in accounting principle			$ 3
Other comprehensive income before reclassifications	(84)	270
Amounts reclassified from accumulated other comprehensive income	15	5
Other comprehensive income	(66)	275
Balance at end of period	(3,307)	(3,125)
Defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	(1,583)	(1,524)
Cumulative effect of change in accounting principle
Other comprehensive income before reclassifications	(4)	20
Amounts reclassified from accumulated other comprehensive income	33	20
Other comprehensive income	29	40
Balance at end of period	(1,554)	(1,484)
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	(1,681)	(1,879)
Cumulative effect of change in accounting principle
Other comprehensive income before reclassifications	(71)	223
Amounts reclassified from accumulated other comprehensive income
Other comprehensive income	(71)	223
Balance at end of period	(1,752)	(1,656)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	23	3
Cumulative effect of change in accounting principle			$ 3
Other comprehensive income before reclassifications	(9)	27
Amounts reclassified from accumulated other comprehensive income	(18)	(15)
Other comprehensive income	(24)	12
Balance at end of period	$ (1)	$ 15