Accumulated Other Comprehensive Income (Loss) - After-tax Changes by Component (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (3,422)	$ (3,778)
Amounts arising during the period	95	(30)
Reclassifications from accumulated other comprehensive income	48	31
Net other comprehensive income during the period	143	1
Balance at end of period	(3,279)	(3,777)
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,089)	(1,627)
Amounts arising during the period	116	(24)
Reclassifications from accumulated other comprehensive income	0	0
Net other comprehensive income during the period	116	(24)
Balance at end of period	(973)	(1,651)
Qualifying Derivatives Unrealized Gains (Losses) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(13)	(7)
Amounts arising during the period	(20)	(6)
Reclassifications from accumulated other comprehensive income	12	(1)
Net other comprehensive income during the period	(8)	(7)
Balance at end of period	(21)	(14)
Equity Investees Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	0	(1)
Amounts arising during the period	0	0
Reclassifications from accumulated other comprehensive income	0	0
Net other comprehensive income during the period	0	0
Balance at end of period	0	(1)
Defined Benefit Plans Prior Service Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(7)	(8)
Amounts arising during the period	0	0
Reclassifications from accumulated other comprehensive income	0	0
Net other comprehensive income during the period	0	0
Balance at end of period	(7)	(8)
Defined Benefit Plans Actuarial Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(2,313)	(2,135)
Amounts arising during the period	(1)	0
Reclassifications from accumulated other comprehensive income	36	32
Net other comprehensive income during the period	35	32
Balance at end of period	$ (2,278)	$ (2,103)