Leases - Supplemental Balance Sheet Information Related to Leases (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Operating Leases:
Operating Lease ROU Assets	$ 667	$ 642
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	us-gaap:OtherAssets	us-gaap:OtherAssets
Finance Leases:
Property and equipment	$ 9,536	$ 9,352
Less accumulated depreciation and amortization	(4,168)	(4,030)
Property and equipment, net	5,368	5,322
Current finance lease liabilities	$ 9	$ 5
Finance Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	us-gaap:DebtCurrent	us-gaap:DebtCurrent
Long-term finance lease liabilities	$ 112	$ 74
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Long-term debt	Long-term debt
Finance Lease ROU Assets
Finance Leases:
Property and equipment	$ 195	$ 150
Less accumulated depreciation and amortization	(51)	(46)
Property and equipment, net	144	104
Finance Lease ROU Assets | Land and Improvements
Finance Leases:
Property and equipment	8	8
Finance Lease ROU Assets | Building and Improvements
Finance Leases:
Property and equipment	173	134
Finance Lease ROU Assets | Equipment and Fixtures
Finance Leases:
Property and equipment	$ 14	$ 8