Leases (Supplemental Balance Sheet Information Related To Leases) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Supplemental Balance Sheet Information Related To Leases [Line Items]
Operating right-of-use assets	$ 215	$ 204
Finance right-of-use assets	143	167
Operating lease liabilities	223	211
Finance lease liabilities	$ 145	$ 167
Operating leases, Weighted-average remaining lease term	7 years 9 months	7 years 6 months 14 days
Operating leases, Weighted-average discount rate	8.26%	8.25%
Finance lease, Weighted-average remaining lease term	8 years 11 months 15 days	9 years 1 month 6 days
Finance lease, Weighted-average discount rate	8.13%	7.98%
Other Current Liabilities [Member]
Supplemental Balance Sheet Information Related To Leases [Line Items]
Finance lease liabilities, Current	$ 21	$ 25
Current operating lease liabilities	48	44
Other Noncurrent Liabilities [Member]
Supplemental Balance Sheet Information Related To Leases [Line Items]
Financed lease liabilities, Noncurrent	124	142
Noncurrent operating lease liabilities	$ 175	$ 167