Accumulated Other Comprehensive Income - Components (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 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	156	(84)
Amounts reclassified from accumulated other comprehensive income	78	15
Other comprehensive income (loss)	234	(66)
Balance at end of period	(3,140)	(3,307)
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
Other comprehensive income before reclassifications	140	(4)
Amounts reclassified from accumulated other comprehensive income	55	33
Other comprehensive income (loss)	195	29
Balance at end of period	(1,338)	(1,554)
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
Other comprehensive income before reclassifications	40	(71)
Amounts reclassified from accumulated other comprehensive income
Other comprehensive income (loss)	40	(71)
Balance at end of period	(1,777)	(1,752)
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	(24)	(9)
Amounts reclassified from accumulated other comprehensive income	23	(18)
Other comprehensive income (loss)	(1)	(24)
Balance at end of period	$ (25)	$ (1)