Leases - Supplemental Balance Sheet Information Related to Leases (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Operating Leases:
Operating Lease ROU Assets	$ 633	$ 607
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Finance Leases:
Property and equipment	$ 9,293	$ 9,653
Less accumulated depreciation and amortization	(4,003)	(4,045)
Property and equipment, net	5,290	5,608
Current finance lease liabilities	$ 5	$ 6
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent
Long-term finance lease liabilities	$ 75	$ 107
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtNoncurrent	us-gaap:LongTermDebtNoncurrent
Finance Lease ROU Assets [Member]
Finance Leases:
Property and equipment	$ 151	$ 173
Less accumulated depreciation and amortization	(45)	(56)
Property and equipment, net	106	117
Finance Lease ROU Assets [Member] | Land and Improvements [Member]
Finance Leases:
Property and equipment	8	8
Finance Lease ROU Assets [Member] | Building and Building Improvements [Member]
Finance Leases:
Property and equipment	134	154
Finance Lease ROU Assets [Member] | Equipment and Fixtures [Member]
Finance Leases:
Property and equipment	$ 9	$ 11