Long-Term Debt (Senior Unsecured Debt) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal Outstanding		$ 18,016	$ 18,178
Senior Notes and Debentures [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 13,606	15,150
Senior Notes and Debentures [Member] | Senior Note Due 4/15/2017 [Member]
Debt Instrument [Line Items]
Principal Outstanding			$ 210
Interest Rate		8.25%	8.25%
Debt instrument, maturity date		Apr. 15, 2017
Senior Notes and Debentures [Member] | Senior Note Due 10/1/2018 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 578	$ 583
Interest Rate		8.125%	8.125%
Debt instrument, maturity date		Oct. 01, 2018
Senior Notes and Debentures [Member] | Senior Note Due 3/15/2019 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 428	$ 434
Interest Rate		7.125%	7.125%
Debt instrument, maturity date		Mar. 15, 2019
Senior Notes and Debentures [Member] | Senior Note Due 4/15/2020 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 619	$ 1,169
Interest Rate		8.50%	8.50%
Debt instrument, maturity date		Apr. 15, 2020
Senior Notes and Debentures [Member] | Senior Note Due 9/15/2020 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 303	$ 1,066
Interest Rate		8.875%	8.875%
Debt instrument, maturity date		Sep. 15, 2020
Senior Notes and Debentures [Member] | Senior Note Due 7/1/2021 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 490	$ 500
Interest Rate		9.25%	9.25%
Debt instrument, maturity date		Jul. 01, 2021
Senior Notes and Debentures [Member] | Senior Note Due 9/15/2021 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 775	$ 775
Interest Rate		6.25%	6.25%
Debt instrument, maturity date		Sep. 15, 2021
Senior Notes and Debentures [Member] | Senior Note Due 4/15/2022 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 500	$ 500
Interest Rate		8.75%	8.75%
Debt instrument, maturity date		Apr. 15, 2022
Senior Notes and Debentures [Member] | Senior Note Due 9/15/2022 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 2,188	$ 2,188
Interest Rate		10.50%	10.50%
Debt instrument, maturity date		Sep. 15, 2022
Senior Notes and Debentures [Member] | Senior Note Due 1/15/2023 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 850	$ 850
Interest Rate		7.125%	7.125%
Debt instrument, maturity date		Jan. 15, 2023
Senior Notes and Debentures [Member] | Senior Note Due 4/15/2024 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 750	$ 750
Interest Rate		7.625%	7.625%
Debt instrument, maturity date		Apr. 15, 2024
Senior Notes and Debentures [Member] | Senior Note Due 1/15/2025 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 775	$ 775
Interest Rate		6.875%	6.875%
Debt instrument, maturity date		Jan. 15, 2025
Senior Notes and Debentures [Member] | Senior Note Due 9/15/2025 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 3,600	$ 3,600
Interest Rate		11.00%	11.00%
Debt instrument, maturity date		Sep. 15, 2025
Senior Notes and Debentures [Member] | Debenture Due 11/1/2025 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 138	$ 138
Interest Rate		7.00%	7.00%
Debt instrument, maturity date		Nov. 01, 2025
Senior Notes and Debentures [Member] | Debenture Due 8/15/2026 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 2	$ 2
Interest Rate		6.80%	6.80%
Debt instrument, maturity date		Aug. 15, 2026
Senior Notes and Debentures [Member] | Senior Note Due 1/15/2027 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 346	$ 346
Interest Rate		7.875%	7.875%
Debt instrument, maturity date		Jan. 15, 2027
Senior Notes and Debentures [Member] | Senior Note Due 8/15/2031 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 945	$ 945
Interest Rate		9.00%	9.00%
Debt instrument, maturity date		Aug. 15, 2031
Senior Notes and Debentures [Member] | Debenture Due 10/1/2034 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 1	$ 1
Interest Rate		7.68%	7.68%
Debt instrument, maturity date		Oct. 01, 2034
Senior Notes and Debentures [Member] | Debenture Due 7/1/2035 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 125	$ 125
Interest Rate		7.45%	7.45%
Debt instrument, maturity date		Jul. 01, 2035
Senior Notes and Debentures [Member] | Debenture Due 10/1/2046 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 193	$ 193
Interest Rate		7.05%	7.05%
Debt instrument, maturity date		Oct. 01, 2046
Senior Secured Debt [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 3,537	$ 2,151
Senior Secured Debt [Member] | Senior Debt Due 10/24/2019 [Member]
Debt Instrument [Line Items]
Principal Outstanding	[1]	$ 254	$ 280
Interest Rate	[1]	5.115%	4.145%
Debt instrument, maturity date	[1]	Oct. 24, 2019
Senior Secured Debt [Member] | Senior Debt Due 3/31/2021 [Member]
Debt Instrument [Line Items]
Principal Outstanding	[2]	$ 1,503	$ 1,564
Interest Rate	[2]	3.99%	3.27%
Debt instrument, maturity date	[2]	Mar. 31, 2021
Senior Secured Debt [Member] | Senior Debt Due 10/12/2021
Debt Instrument [Line Items]
Principal Outstanding	[3]	$ 284	$ 307
Interest Rate	[3]	5.115%	4.145%
Debt instrument, maturity date	[3]	Oct. 12, 2021
Senior Secured Debt [Member] | Senior Debt Due 6/15/2024
Debt Instrument [Line Items]
Principal Outstanding	[4]	$ 1,496
Interest Rate	[4]	4.99%
Debt instrument, maturity date	[4]	Jun. 15, 2024
Subsidiary Debt [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 850	$ 850
Subsidiary Debt [Member] | Subsidiary Senior Note Due 5/15/2027 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 200	$ 200
Interest Rate		6.75%	6.75%
Debt instrument, maturity date		May 15, 2027
Subsidiary Debt [Member] | Subsidiary Senior Note Due 2/1/2028 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 300	$ 300
Interest Rate		6.86%	6.86%
Debt instrument, maturity date		Feb. 01, 2028
Subsidiary Debt [Member] | Subsidiary Senior Note Due 2/15/2028 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 200	$ 200
Interest Rate		6.73%	6.73%
Debt instrument, maturity date		Feb. 15, 2028
Subsidiary Debt [Member] | Subsidiary Senior Note Due 10/15/2029 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 50	$ 50
Interest Rate		8.40%	8.40%
Debt instrument, maturity date		Oct. 15, 2029
Subsidiary Debt [Member] | Subsidiary Senior Note Due 11/15/2031 [Member]
Debt Instrument [Line Items]
Principal Outstanding		$ 100	$ 100
Interest Rate		8.50%	8.50%
Debt instrument, maturity date		Nov. 15, 2031
Senior Secured Debt, Senior Unsecured Debt And Subsidiary Debentures [Member]
Debt Instrument [Line Items]
Principal Outstanding	[5]	$ 17,993	$ 18,151
Weighted average interest rate	[5]	8.10%	8.30%

[1] Represents borrowings under the 2014 CoBank Credit Agreement, as defined below.

[2] Represents borrowings under the JPM Credit Agreement Term Loan A, as defined below.

[3] Represents borrowings under the 2016 CoBank Credit Agreement, as defined below.

[4] Represents borrowings under the JPM Credit Agreement Term Loan B, as defined below.

[5] Interest rate represents a weighted average of the stated interest rates of multiple issuances.