Financial and Other Derivative Instruments (Details Textuals) (USD $)			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 FTR [Member]	Mar. 31, 2012 FTR [Member]
Derivative [Line Items]
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ 1,000,000
Cash Collateral Remaiining Balance After Netting	0
Letters of Credit Issued	9,000,000	63,000,000
Cash Collateral Posted Net of Cash Collateral Received	53,000,000
Collateral Already Posted, Aggregate Fair Value		20,000,000
Cash Collateral Paid or Received for Derivative Assets	4,000,000	0
Cash Collateral Paid or Received for Derivative Liabilities	56,000,000
Cash Collateral Received	3,000,000	24,000,000
Cash Collateral Paid	4,000,000	4,000,000
Derivative Instruments Recoverable Through PSCR Mechanism			1,000,000	1,000,000
Value of Transactions Company Would Have Been Exposed to if Credit Rating Below Investment Grade	$ 306,000,000