Lease Obligations - Lessee - Lease Assets and Lease Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Leases [Abstract]
Operating lease right-of-use assets	$ 425	$ 423
Operating lease right-of-use assets [Extensible List]	Other assets	Other assets
Finance lease right-of-use assets	$ 144	$ 129
Finance lease, right-of-use assets [Extensible List]	Property and equipment	Property and equipment
Total right-of-use assets	$ 569	$ 552
Operating lease liabilities	$ 523	$ 527
Operating lease liabilities [Extensible List]	us-gaap:AccruedLiabilitiesMember	us-gaap:AccruedLiabilitiesMember
Finance lease liabilities	$ 146	$ 130
Finance lease liabilities [Extensible List]	Long-term debt	Long-term debt
Total lease liabilities	$ 669	$ 657
Operating lease right of use asset, accumulated amortization	169	152
Finance lease right of use asset, accumulated amortization	$ 38	$ 29