Leases - Lease Position (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets
Operating lease assets	$ 5,733	$ 5,627
Finance lease assets	1,002	1,062
Total lease assets	$ 6,735	$ 6,689
Finance lease asset, balance sheet	us-gaap:PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization	us-gaap:PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization
Liabilities
Current operating lease liabilities	$ 678	$ 801
Current finance lease liabilities	289	233
Noncurrent operating lease liabilities	5,713	5,294
Noncurrent finance lease liabilities	894	821
Total lease liabilities	$ 7,574	$ 7,149
Operating leases, weighted-average remaining lease term	12 years	12 years
Finance leases, weighted-average remaining lease term	5 years	5 years
Operating leases, weighted-average discount rate	4.88%	3.73%
Finance leases, weighted-average discount rate	3.61%	3.46%
Finance lease liability, current, balance sheet	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent
Finance lease liability, noncurrent, balance sheet	us-gaap:LongTermDebtAndCapitalLeaseObligations	us-gaap:LongTermDebtAndCapitalLeaseObligations