Long-Term Debt (Schedule Of Unsecured Debt) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Mar. 15, 2019	Dec. 31, 2018	Mar. 19, 2018
Debt Instrument [Line Items]
Principal Outstanding	$ 16,769	$ 17,516
Less: Debt subject to compromise	(10,949)
Total Debt	$ 5,820	17,516
Interest rate	5.944%
Weighted average interest rate	8.486%
Secured Debt [Member]
Debt Instrument [Line Items]
Principal Outstanding	$ 4,964	5,711
Secured Debt [Member] | Revolver Due 2/27/2024 [Member]
Debt Instrument [Line Items]
Maturity date	Feb. 27, 2024
Principal Outstanding		$ 749
Interest rate		4.76%
Secured Debt [Member] | Term Loan Due 06/15/2024 [Member]
Debt Instrument [Line Items]
Maturity date	Jun. 15, 2024
Principal Outstanding		$ 1,699
Interest rate		5.55%
Secured Debt [Member] | First Lien Notes Due 4/1/2027 [Member]
Debt Instrument [Line Items]
Maturity date	Apr. 01, 2027
Principal Outstanding		$ 1,650
Interest rate		8.00%	8.00%
Secured Debt [Member] | Second Lien Notes Due 4/1/2026 [Member]
Debt Instrument [Line Items]
Maturity date	Apr. 01, 2026
Principal Outstanding		$ 1,600
Interest rate	8.50%	8.50%			8.50%
Secured Debt [Member] | DIP-to-Exit term loan due 10/8/2027 [Member]
Debt Instrument [Line Items]
Maturity date	Oct. 08, 2027
Principal Outstanding	$ 1,250
Interest rate	5.75%
Secured Debt [Member] | DIP-to-Exit First Lien Notes Due 10/15/2027 [Member]
Debt Instrument [Line Items]
Maturity date	Oct. 15, 2027
Principal Outstanding	$ 1,150
Interest rate	5.875%
Secured Debt [Member] | DIP-to-Exit First Lien Notes Due 5/1/2028 [Member]
Debt Instrument [Line Items]
Maturity date	May 01, 2028
Principal Outstanding	$ 1,550
Interest rate	5.00%
Secured Debt [Member] | DIP-to-Exit Second Lien Notes Due 5/1/2029 [Member]
Debt Instrument [Line Items]
Maturity date	May 01, 2029
Principal Outstanding	$ 1,000
Interest rate	6.75%
Secured Debt [Member] | IDRB due 5/1/2030 [Member]
Debt Instrument [Line Items]
Maturity date	May 01, 2030
Principal Outstanding	$ 14	$ 13
Interest rate	6.20%	6.20%
Unsecured Debt [Member]
Debt Instrument [Line Items]
Principal Outstanding	$ 10,949	$ 10,949
Unsecured Debt [Member] | Senior Note Due 4/15/2020 [Member]
Debt Instrument [Line Items]
Maturity date	Apr. 15, 2020
Principal Outstanding	$ 172	$ 172
Interest rate	8.50%	8.50%		8.50%
Unsecured Debt [Member] | Senior Note Due 9/15/2020 [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 15, 2020
Principal Outstanding	$ 55	$ 55
Interest rate	8.875%	8.875%
Unsecured Debt [Member] | Senior Note Due 7/1/2021 [Member]
Debt Instrument [Line Items]
Maturity date	Jul. 01, 2021
Principal Outstanding	$ 89	$ 89
Interest rate	9.25%	9.25%
Unsecured Debt [Member] | Senior Note Due 9/15/2021 [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 15, 2021
Principal Outstanding	$ 220	$ 220
Interest rate	6.25%	6.25%
Unsecured Debt [Member] | Senior Note Due 4/15/2022 [Member]
Debt Instrument [Line Items]
Maturity date	Apr. 15, 2022
Principal Outstanding	$ 500	$ 500
Interest rate	8.75%	8.75%
Unsecured Debt [Member] | Senior Note Due 9/15/2022 [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 15, 2022
Principal Outstanding	$ 2,188	$ 2,188
Interest rate	10.50%	10.50%
Unsecured Debt [Member] | Senior Note Due 1/15/2023 [Member]
Debt Instrument [Line Items]
Maturity date	Jan. 15, 2023
Principal Outstanding	$ 850	$ 850
Interest rate	7.125%	7.125%
Unsecured Debt [Member] | Senior Note Due 4/15/2024 [Member]
Debt Instrument [Line Items]
Maturity date	Apr. 15, 2024
Principal Outstanding	$ 750	$ 750
Interest rate	7.625%	7.625%
Unsecured Debt [Member] | Senior Note Due 1/15/2025 [Member]
Debt Instrument [Line Items]
Maturity date	Jan. 15, 2025
Principal Outstanding	$ 775	$ 775
Interest rate	6.875%	6.875%
Unsecured Debt [Member] | Senior Note Due 9/15/2025 [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 15, 2025
Principal Outstanding	$ 3,600	$ 3,600
Interest rate	11.00%	11.00%
Unsecured Debt [Member] | Debentures Due 11/1/2025 [Member]
Debt Instrument [Line Items]
Maturity date	Nov. 01, 2025
Principal Outstanding	$ 138	$ 138
Interest rate	7.00%	7.00%
Unsecured Debt [Member] | Debentures Due 8/15/2026 [Member]
Debt Instrument [Line Items]
Maturity date	Aug. 15, 2026
Principal Outstanding	$ 2	$ 2
Interest rate	6.80%	6.80%
Unsecured Debt [Member] | Senior Note Due 1/15/2027 [Member]
Debt Instrument [Line Items]
Maturity date	Jan. 15, 2027
Principal Outstanding	$ 346	$ 346
Interest rate	7.875%	7.875%
Unsecured Debt [Member] | Senior Note Due 8/15/2031 [Member]
Debt Instrument [Line Items]
Maturity date	Aug. 15, 2031
Principal Outstanding	$ 945	$ 945
Interest rate	9.00%	9.00%
Unsecured Debt [Member] | Debentures Due 10/1/2034 [Member]
Debt Instrument [Line Items]
Maturity date	Oct. 01, 2034
Principal Outstanding	$ 1	$ 1
Interest rate	7.68%	7.68%
Unsecured Debt [Member] | Debentures Due 7/1/2035 [Member]
Debt Instrument [Line Items]
Maturity date	Jul. 01, 2035
Principal Outstanding	$ 125	$ 125
Interest rate	7.45%	7.45%
Unsecured Debt [Member] | Debentures Due 10/1/2046 [Member]
Debt Instrument [Line Items]
Maturity date	Oct. 01, 2046
Principal Outstanding	$ 193	$ 193
Interest rate	7.05%	7.05%
Secured Subsidiary Debt [Member]
Debt Instrument [Line Items]
Principal Outstanding	$ 106	$ 106
Secured Subsidiary Debt [Member] | Debentures Due 11/15/2031 [Member]
Debt Instrument [Line Items]
Maturity date	Nov. 15, 2031
Principal Outstanding	$ 100	$ 100
Interest rate	8.50%	8.50%
Secured Subsidiary Debt [Member] | RUS Loan Contracts Due 1/3/2028 [Member]
Debt Instrument [Line Items]
Maturity date	Jan. 03, 2028
Principal Outstanding	$ 6	$ 6
Interest rate	6.154%	6.154%
Unsecured Subsidiary Debt [Member]
Debt Instrument [Line Items]
Principal Outstanding	$ 750	$ 750
Unsecured Subsidiary Debt [Member] | Debentures due 5/15/2027 [Member]
Debt Instrument [Line Items]
Maturity date	May 15, 2027
Principal Outstanding	$ 200	$ 200
Interest rate	6.75%	6.75%
Unsecured Subsidiary Debt [Member] | Debentures Due 2/1/2028 [Member]
Debt Instrument [Line Items]
Maturity date	Feb. 01, 2028
Principal Outstanding	$ 300	$ 300
Interest rate	6.86%	6.86%
Unsecured Subsidiary Debt [Member] | Debentures Due 2/15/2028 [Member]
Debt Instrument [Line Items]
Maturity date	Feb. 15, 2028
Principal Outstanding	$ 200	$ 200
Interest rate	6.73%	6.73%
Unsecured Subsidiary Debt [Member] | Debentures Due 10/15/2029 [Member]
Debt Instrument [Line Items]
Maturity date	Oct. 15, 2029
Principal Outstanding	$ 50	$ 50
Interest rate	8.40%	8.40%
Senior Secured Debt, Senior Unsecured Debt And Subsidiary Debentures [Member]
Debt Instrument [Line Items]
Principal Outstanding	$ 16,769	$ 17,516
Weighted average interest rate	8.188%	8.486%