Accumulated Other Comprehensive Loss - Reclassifications (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	$ 1,668	$ 2,021	$ 2,177
Other (expense) income, net	(76)	(19)	44
Total before tax	(782)	(4,766)	(1,563)
Income tax expense	(450)	(503)	(170)
Net of tax	(1,232)	(5,269)	$ (1,733)
Reclassification out of Accumulated Other Comprehensive Income | Interest rate swap
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	62	2
Income tax expense	(16)
Net of tax	46
Reclassification out of Accumulated Other Comprehensive Income | Net actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (expense) income, net	203	224
Reclassification out of Accumulated Other Comprehensive Income | Prior service cost
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (expense) income, net	7	8
Reclassification out of Accumulated Other Comprehensive Income | Curtailment loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (expense) income, net	4
Reclassification out of Accumulated Other Comprehensive Income | Defined benefit plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	214	234
Income tax expense	(52)	(57)
Net of tax	$ 162	$ 177