Long-Term Debt (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Securitization Bonds [Member]	Dec. 31, 2012 Mortgage Bonds 3 [Member]	Dec. 31, 2012 Mortgage Bonds 4 [Member]	Dec. 31, 2012 Tax-Exempt Revenue Bonds 2 [Member]	Dec. 31, 2012 Secured Medium Term Notes [Member]	Dec. 31, 2012 Senior Notes 1 [Member]	Dec. 31, 2012 Tax Exempt Revenue Bonds [Member]	Dec. 31, 2012 Tax-Exempt Revenue Bonds 1 [Member]	Dec. 31, 2012 Long-term Debt [Member]
Extinguishment of Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		6.42%	7.90%	8.36%	5.25%	7.06%	5.20%	3.05%	5.45%
Extinguishment of Debt, Amount	$ 639	$ 164	$ 10	$ 3	$ 56	$ 40	$ 225	$ 65	$ 64	$ 12