Accumulated Other Comprehensive Income - Components (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	$ (3,374)	$ (3,241)
Cumulative effect of change in accounting principle			$ 3
Other comprehensive income before reclassifications	78	55
Amounts reclassified from accumulated other comprehensive income	7	5
Other comprehensive income (loss)	85	63
Balance at end of period	(3,289)	(3,178)
Defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	(1,533)	(1,583)
Cumulative effect of change in accounting principle			0
Other comprehensive income before reclassifications	13	0
Amounts reclassified from accumulated other comprehensive income	1	11
Other comprehensive income (loss)	14	11
Balance at end of period	(1,519)	(1,572)
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	(1,817)	(1,681)
Cumulative effect of change in accounting principle			0
Other comprehensive income before reclassifications	59	81
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income (loss)	59	81
Balance at end of period	(1,758)	(1,600)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	(24)	23
Cumulative effect of change in accounting principle			$ 3
Other comprehensive income before reclassifications	6	(26)
Amounts reclassified from accumulated other comprehensive income	6	(6)
Other comprehensive income (loss)	12	(29)
Balance at end of period	$ (12)	$ (6)