Credit Risk (Narrative) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Gross credit exposure	$ 311
Additional collateral to be posted if the credit-related contingent features were triggered	112	88
Collateral already posted	92	54
Letters of credit included in the collateral posted	87	19
Fair value of all derivative instruments not fully collateralized	$ 247	$ 210
Investment Grade [Member] \| Internal Credit Rating [Member]
Concentration of credit risk	80.00%
Investment Grade [Member] \| Counterparty, A [Member]
Concentration of credit risk	11.00%

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
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-Paragraph 44D
-Subparagraph c
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

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Reflects the maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

+ References

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-Publisher FASB
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-Subparagraph b
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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

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The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

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The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for letter of credit arrangements, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

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