Leases (Supplemental Balance Sheet Information Related to Leases) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Operating Leases:
Operating Lease ROU Assets	$ 634	$ 607
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent
Finance Leases:
Property and equipment	$ 9,618	$ 9,653
Less accumulated depreciation and amortization	(4,096)	(4,045)
Property and equipment, net	5,522	5,608
Current finance lease liabilities	$ 6	$ 6
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent
Long-term finance lease liabilities	$ 122	$ 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	$ 187	$ 173
Less accumulated depreciation and amortization	(58)	(56)
Property and equipment, net	129	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	168	154
Finance Lease ROU Assets [Member] | Equipment and Fixtures [Member]
Finance Leases:
Property and equipment	$ 11	$ 11