Accumulated Other Comprehensive Income (Loss) - After-tax Changes by Component (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (3,964)	$ (3,422)
Amounts arising during the period	272	95
Reclassifications from AOCI	45	48
Net OCI during the period	317	143
Balance at end of period	(3,647)	(3,279)
Foreign currency translation adjustments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(1,533)	(1,089)
Amounts arising during the period	294	116
Reclassifications from AOCI	0	0
Net OCI during the period	294	116
Balance at end of period	(1,239)	(973)
Unrealized gains (losses) on qualifying derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(7)	(13)
Amounts arising during the period	(19)	(20)
Reclassifications from AOCI	24	12
Net OCI during the period	5	(8)
Balance at end of period	(2)	(21)
Defined benefit plans Prior service costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(19)	(7)
Amounts arising during the period	0	0
Reclassifications from AOCI	0	0
Net OCI during the period	0	0
Balance at end of period	(19)	(7)
Defined benefit plans Actuarial gain (loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(2,405)	(2,313)
Amounts arising during the period	(3)	(1)
Reclassifications from AOCI	21	36
Net OCI during the period	18	35
Balance at end of period	$ (2,387)	$ (2,278)