Accumulated Other Comprehensive Income - Components (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 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	47	(66)
Amounts reclassified from accumulated other comprehensive income	53	10
Other comprehensive income (loss)	100	(53)
Balance at end of period	(3,274)	(3,294)
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	7	0
Amounts reclassified from accumulated other comprehensive income	40	22
Other comprehensive income (loss)	47	22
Balance at end of period	(1,486)	(1,561)
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	50	(61)
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income (loss)	50	(61)
Balance at end of period	(1,767)	(1,742)
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	(10)	(5)
Amounts reclassified from accumulated other comprehensive income	13	(12)
Other comprehensive income (loss)	3	(14)
Balance at end of period	$ (21)	$ 9