Long-Term Debt (Long-Term Debt) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt, beginning balance		$ 18,178
Payments and Retirements		(1,662)
New Borrowings		1,500
Long-term debt, ending balance		18,016
Long-term debt		18,178	$ 18,016	$ 18,178
Less: Debt Issuance Cost			(190)	(209)
Less: Debt Premium/(Discount)			56	46
Less: Current Portion			(166)	(363)
Long-term Debt, Excluding Current Maturities			$ 17,604	17,560
Weighted average interest rate	[1]		8.36%
Senior And Subsidiary Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term debt, beginning balance		15,900
Payments and Retirements		(1,544)
Long-term debt, ending balance		14,356
Long-term debt		15,900	$ 14,356	15,900
Weighted average interest rate	[1]		9.22%
Senior Secured Debt [Member]
Debt Instrument [Line Items]
Long-term debt, beginning balance		2,151
Payments and Retirements		(114)
New Borrowings		1,500
Long-term debt, ending balance		3,537
Long-term debt		2,151	$ 3,537	2,151
Weighted average interest rate	[1]		4.90%
Secured Subsidiary Debt [Member]
Debt Instrument [Line Items]
Long-term debt, beginning balance		100
Payments and Retirements
New Borrowings
Long-term debt, ending balance		100
Long-term debt		100	$ 100	100
Weighted average interest rate	[1]		8.50%
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term debt, beginning balance		19
Payments and Retirements		(3)
Long-term debt, ending balance		16
Long-term debt		19	$ 16	19
Weighted average interest rate	[1]		5.25%
Rural Utilities Service Loan Contracts [Member]
Debt Instrument [Line Items]
Long-term debt, beginning balance		8
Payments and Retirements		(1)
Long-term debt, ending balance		7
Long-term debt		$ 8	$ 7	$ 8
Weighted average interest rate	[1]		6.15%

[1] Interest rate includes amortization of debt issuance costs and debt premiums or discounts. The interest rates at September 30, 2017 represent a weighted average of multiple issuances.