Lease Commitments - Lease Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Assets:
Operating lease assets	$ 1,129	$ 1,411
Finance lease assets	$ 338	397
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization
Total leased assets	$ 1,467	1,808
Liabilities:
Operating lease liabilities, current	420	579
Finance lease liabilities, current	$ 34	51
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:DebtCurrent
Operating lease liabilities, non-current	$ 740	872
Finance lease liabilities, non-current	$ 301	347
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtNoncurrent
Total lease liabilities	$ 1,495	$ 1,849