Lease Obligations - Lessee - Lease Assets and Lease Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease right-of-use assets	$ 423	$ 473
Operating lease right-of-use assets [Extensible List]	us-gaap:OtherAssets	us-gaap:OtherAssets
Finance lease right-of-use assets	$ 129	$ 34
Finance lease, right-of-use assets [Extensible List]	us-gaap:PropertyPlantAndEquipmentNet	us-gaap:PropertyPlantAndEquipmentNet
Total right-of-use assets	$ 552	$ 507
Operating lease liabilities	$ 527	$ 555
Operating lease liabilities [Extensible List]	us-gaap:AccruedLiabilitiesCurrentAndNoncurrent	us-gaap:AccruedLiabilitiesCurrentAndNoncurrent
Finance lease liabilities	$ 130	$ 35
Finance lease liabilities [Extensible List]	us-gaap:LongTermDebt	us-gaap:LongTermDebt
Total lease liabilities	$ 657	$ 590
Operating lease right of use asset, accumulated amortization	152	75
Finance lease right of use asset, accumulated amortization	$ 29	$ 27