Long-Term Debt (Long-Term Debt) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt, beginning balance	$ 18,178
Payments and Retirements	(1,814)
New Borrowings	1,500
Long-term debt, ending balance	(18,178)	$ (17,863)	$ (18,178)
Less: Debt Issuance Costs		(183)	(209)
Less: Debt Discount/Premium		(54)	(46)
Less: Current Portion		(656)	(363)
Total Long-Term Debt		$ 16,970	17,560
Weighted average interest rate		8.44%
Senior and Subsidiary Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term debt, beginning balance	15,900
Payments and Retirements	(1,655)
Long-term debt, ending balance	(15,900)	$ (14,245)	(15,900)
Weighted average interest rate		9.22%
Senior Secured Debt [Member]
Debt Instrument [Line Items]
Long-term debt, beginning balance	2,151
Payments and Retirements	(155)
New Borrowings	1,500
Long-term debt, ending balance	(2,151)	$ (3,496)	(2,151)
Weighted average interest rate		5.24%
Secured Subsidiary Debt [Member]
Debt Instrument [Line Items]
Long-term debt, beginning balance	100
Long-term debt, ending balance	(100)	$ (100)	(100)
Weighted average interest rate		8.50%
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term debt, beginning balance	19
Payments and Retirements	(3)
Long-term debt, ending balance	(19)	$ (16)	(19)
Weighted average interest rate		5.56%
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	$ (8)	$ (7)	$ (8)
Weighted average interest rate		6.15%