Long-Term Debt (Senior Unsecured Debt) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Debt Instrument [Line Items]
Principal Outstanding	$ 9,655		$ 9,781
Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Principal Outstanding	9,622		9,750
Weighted average interest rate	7.49%	[1]	7.45%	[1]
Senior Notes and Debentures [Member] | Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Principal Outstanding	9,372		9,500
Senior Notes and Debentures [Member] | Senior Unsecured Debt [Member] | Senior Note Due 3/15/2015 [Member]
Debt Instrument [Line Items]
Principal Outstanding			105
Interest Rate			6.625%
Debt instrument, maturity date	Mar. 15, 2015
Senior Notes and Debentures [Member] | Senior Unsecured Debt [Member] | Senior Note Due 4/15/2015 [Member]
Debt Instrument [Line Items]
Principal Outstanding	97		97
Interest Rate	7.875%		7.875%
Debt instrument, maturity date	Apr. 15, 2015
Senior Notes and Debentures [Member] | Senior Unsecured Debt [Member] | Senior Note Due 10/14/2016 [Member]
Debt Instrument [Line Items]
Principal Outstanding	388	[2]	402	[2]
Interest Rate	3.555%	[2]	3.045%	[2]
Debt instrument, maturity date	Oct. 14, 2016	[2]
Senior Notes and Debentures [Member] | Senior Unsecured Debt [Member] | Senior Note Due 4/15/2017 [Member]
Debt Instrument [Line Items]
Principal Outstanding	607		607
Interest Rate	8.25%		8.25%
Debt instrument, maturity date	Apr. 15, 2017
Senior Notes and Debentures [Member] | Senior Unsecured Debt [Member] | Senior Note Due 10/1/2018 [Member]
Debt Instrument [Line Items]
Principal Outstanding	583		583
Interest Rate	8.125%		8.125%
Debt instrument, maturity date	Oct. 01, 2018
Senior Notes and Debentures [Member] | Senior Unsecured Debt [Member] | Senior Note Due 3/15/2019 [Member]
Debt Instrument [Line Items]
Principal Outstanding	434		434
Interest Rate	7.125%		7.125%
Debt instrument, maturity date	Mar. 15, 2019
Senior Notes and Debentures [Member] | Senior Unsecured Debt [Member] | Senior Note Due 10/24/2019 [Member]
Debt Instrument [Line Items]
Principal Outstanding	341	[3]	350	[3]
Interest Rate	3.555%	[3]	3.545%	[3]
Debt instrument, maturity date	Oct. 24, 2019	[3]
Senior Notes and Debentures [Member] | Senior Unsecured Debt [Member] | Senior Note Due 4/15/2020 [Member]
Debt Instrument [Line Items]
Principal Outstanding	1,022		1,022
Interest Rate	8.50%		8.50%
Debt instrument, maturity date	Apr. 15, 2020
Senior Notes and Debentures [Member] | Senior Unsecured Debt [Member] | Senior Note Due 7/1/2021 [Member]
Debt Instrument [Line Items]
Principal Outstanding	500		500
Interest Rate	9.25%		9.25%
Debt instrument, maturity date	Jul. 01, 2021
Senior Notes and Debentures [Member] | Senior Unsecured Debt [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 Unsecured Debt [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 Unsecured Debt [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 Unsecured Debt [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 Unsecured Debt [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 Unsecured Debt [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] | Senior Unsecured Debt [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 Unsecured Debt [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 Unsecured Debt [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] | Senior Unsecured Debt [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] | Senior Unsecured Debt [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] | Senior Unsecured Debt [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
Subsidiary Senior Notes And Debentures [Member] | Senior Unsecured 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 Senior Notes And Debentures [Member] | Senior Unsecured 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

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

[2] Represents borrowings under the 2011 CoBank Credit Agreement, as defined below.

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