Use of Collateral (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Use of Collateral [Abstract]
Margin deposits	$ 196	[1]	$ 140	[1]
Natural gas and power prepayments	35		42
Total margin deposits and natural gas and power prepayments with our counterparties	231	[2]	182	[2]
Letters of credit issued	484	[3]	581	[3]
First priority liens under power and natural gas agreements	14		1
First priority liens under interest rate swap agreements	206		318
Total letters of credit and first priority liens with our counterparties	704		900
Margin deposits held by us posted by our counterparties	11	[1],[3]	34	[1],[3]
Letters of credit posted with us by our counterparties	1		0
Total margin deposits and letters of credit posted with us by our counterparties	12		34
Use of Collateral (Textuals) [Abstract]
Margin And Prepayment Amounts Included In Other Assets	20		20
Margin And Prepayment Amounts Included In Margin Deposits And Other Prepaid Expenses	$ 211		$ 162

[1] Balances are subject to master netting arrangements and presented on a gross basis on our Consolidated Balance Sheets. We do not offset fair value amounts recognized for derivative instruments executed with the same counterparty under a master netting arrangement for financial statement presentation.

[2] At December 31, 2012 and 2011, $211 million and $162 million, respectively, were included in margin deposits and other prepaid expense and $20 million and $20 million, respectively, were included in other assets on our Consolidated Balance Sheets.

[3] Included in other current liabilities on our Consolidated Balance Sheets.