Accumulated Other Comprehensive Income - Components (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	55	131
Amounts reclassified from accumulated other comprehensive income	5	8
Other comprehensive income	63	139
Balance at end of period	(3,178)	(3,261)
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
Amounts reclassified from accumulated other comprehensive income	11	9
Other comprehensive income	11	9
Balance at end of period	(1,572)	(1,515)
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	81	110
Amounts reclassified from accumulated other comprehensive income
Other comprehensive income	81	110
Balance at end of period	(1,600)	(1,769)
Gains and losses on cash flow hedges
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	(26)	21
Amounts reclassified from accumulated other comprehensive income	(6)	(1)
Other comprehensive income	(29)	20
Balance at end of period	$ (6)	$ 23