Leases - Supplemental Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2019
Assets
Operating lease assets	$ 1,699	$ 1,686	$ 2,100
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssets	us-gaap:OtherAssets
Finance lease assets	$ 329	$ 252
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentNet	us-gaap:PropertyPlantAndEquipmentNet
Total leased assets	$ 2,028	$ 1,938
Current
Operating	$ 379	$ 416
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OperatingLeaseLiabilityCurrent	us-gaap:OperatingLeaseLiabilityCurrent
Finance	$ 26	$ 35
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent
Noncurrent
Operating	$ 1,405	$ 1,342
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesNoncurrent
Finance	$ 267	$ 185
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligations	us-gaap:LongTermDebtAndCapitalLeaseObligations
Total lease liabilities	$ 2,077	$ 1,978
Weighted-average remaining lease term (years)
Operating leases	6 years 8 months 12 days	7 years 2 months 12 days
Finance leases	12 years 1 month 6 days	11 years 3 months 18 days
Weighted-average discount rate
Operating leases	6.01%	6.46%
Finance leases	4.94%	5.47%