Lease Commitments - Lease Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Assets:
Operating lease assets	$ 1,196	$ 1,411
Finance lease assets	$ 329	$ 397
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization	us-gaap:PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization
Total leased assets	$ 1,525	$ 1,808
Liabilities:
Operating lease liabilities, current	459	579
Current finance leases	$ 33	$ 51
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:DebtCurrent	us-gaap:DebtCurrent
Operating lease liabilities, non-current	$ 786	$ 872
Finance lease liabilities, non-current	$ 292	$ 347
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtNoncurrent	us-gaap:LongTermDebtNoncurrent
Total lease liabilities	$ 1,570	$ 1,849