Risk Management Credit Risks (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2012 notches	Dec. 31, 2011
Credit Derivatives [Line Items]
Restricted Cash and Cash Equivalents, Current	$ 6	$ 0
Number of credit rating notches	2
External Credit Rating, Investment Grade [Member]
Credit Derivatives [Line Items]
Asset position	765
Netting agreement impact	(39)
Cash collateral held	0
Net exposure	726
Collateral liability, margin deposits held by counterparties		10
External Credit Rating, Non Investment Grade [Member]
Credit Derivatives [Line Items]
Collateral liability, margin deposits held by counterparties	8
Interest Rate Swap [Member] \| External Credit Rating, Investment Grade [Member]
Credit Derivatives [Line Items]
Asset position	652
Commodity Contract [Member] \| External Credit Rating, Investment Grade [Member]
Credit Derivatives [Line Items]
Asset position	$ 113

Maximum amount the entity would be obligated to pay on the credit risk derivatives upon occurrence of a credit event.

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The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. For a classified balance sheet represents the current portion only (the noncurrent portion has a separate concept); there is a separate and distinct element for unclassified presentations.

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