Risk Management and Trading Activities (Details 3) (USD $) In Millions	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Changes in financial assets and liabilities at fair value based on level 3 inputs
Balance at beginning of period	$ 372	$ 41	$ 412	$ 84
Unrealized gains (losses)
Included in earnings	(85)	105	19	163
Included in other comprehensive income	13	(18)	30	62
Purchases	415	347	1,932	782
Sales	(536)	(397)	(2,131)	(745)
Settlements	(162)	(1)	(194)	(46)
Transfers into Level 3	(222)	215	(211)	57
Transfers out of Level 3	(39)	12	(101)	(53)
Balance at end of period	$ (244)	$ 304	$ (244)	$ 304

Purchases that have taken place during the period in relation to net assets/liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

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- Definition

Sales that have taken place during the period in relation to net assets/liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ Details

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- Definition

Settlements that have taken place during the period in relation to net assets/liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ Details

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- Definition

Financial assets and liabilities measured at fair value based on level 3 inputs.

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

This element represents (net) transfers into assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

This element represents (net) transfers out of assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period.

+ Details

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- Definition

This element represents total gains or losses for the period (realized and unrealized), arising from assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings.

+ Details

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Period Type:	duration

- Definition

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