Accumulated Other Comprehensive Income - Components (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	$ (3,131)	$ (3,374)
Other comprehensive income / (loss) before reclassifications	(173)	47
Amounts reclassified from accumulated other comprehensive income	103	53
Other comprehensive income	(70)	100
Balance at end of period	(3,201)	(3,274)
Defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	(1,449)	(1,533)
Other comprehensive income / (loss) before reclassifications	158	7
Amounts reclassified from accumulated other comprehensive income	79	40
Other comprehensive income	237	47
Balance at end of period	(1,212)	(1,486)
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	(1,668)	(1,817)
Other comprehensive income / (loss) before reclassifications	(296)	50
Amounts reclassified from accumulated other comprehensive income	0	0
Other comprehensive income	(296)	50
Balance at end of period	(1,964)	(1,767)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	(14)	(24)
Other comprehensive income / (loss) before reclassifications	(35)	(10)
Amounts reclassified from accumulated other comprehensive income	24	13
Other comprehensive income	(11)	3
Balance at end of period	$ (25)	$ (21)