Lease Obligations - Lessee - Lease Assets and Lease Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease right-of-use assets	$ 462	$ 473
Operating lease right-of-use assets [Extensible List]	us-gaap:OtherAssets	us-gaap:OtherAssets
Finance lease right-of-use assets	$ 96	$ 34
Finance lease, right-of-use assets [Extensible List]	us-gaap:PropertyPlantAndEquipmentNet	us-gaap:PropertyPlantAndEquipmentNet
Total right-of-use assets	$ 558	$ 507
Operating lease liabilities	$ 543	$ 555
Operating lease liabilities [Extensible List]	us-gaap:AccruedLiabilitiesCurrentAndNoncurrent	us-gaap:AccruedLiabilitiesCurrentAndNoncurrent
Finance lease liabilities	$ 97	$ 35
Finance lease liabilities [Extensible List]	us-gaap:LongTermDebt	us-gaap:LongTermDebt
Total lease liabilities	$ 640	$ 590
Operating lease right of use asset, accumulated amortization	99	75
Finance lease right of use asset, accumulated amortization	$ 29	$ 27