Financial and Other Derivative Instruments (Details Textuals) (USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ 1,000,000	$ 1,000,000
Cash Collateral Remaiining Balance After Netting	0	0
Letters of Credit Issued	37,000,000	37,000,000	63,000,000
Cash Collateral Posted Net of Cash Collateral Received	2,000,000	2,000,000
Collateral Already Posted, Aggregate Fair Value			20,000,000
Cash Collateral Paid or Received for Derivative Assets	14,000,000	14,000,000	0
Cash Collateral Paid or Received for Derivative Liabilities	19,000,000	19,000,000
Cash Collateral Received	3,000,000	3,000,000	24,000,000
Cash Collateral Paid	6,000,000	6,000,000	4,000,000
Derivative Instruments Recoverable Through PSCR Mechanism	3,000,000	4,000,000
Value of Transactions Company Would Have Been Exposed to if Credit Rating Below Investment Grade	$ 313,000,000	$ 313,000,000