Accounting for Derivative Instruments and Hedging Activities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011	Dec. 31, 2010
Derivative
Notional amount of interest rate derivatives	$ 1,265	$ 2,782
Fair value of the derivative instrument
Derivative assets	3,121	2,722
Derivative liabilities	2,497	2,050
Emissions \| Short Ton
Derivative
Net notional volume buy/(sell) of open derivative transactions	(3,000,000)	0
Coal \| Short Ton
Derivative
Net notional volume buy/(sell) of open derivative transactions	40,000,000	34,000,000
Natural Gas \| MMBtu
Derivative
Net notional volume buy/(sell) of open derivative transactions	(8,000,000)	(175,000,000)
Oil \| Barrel
Derivative
Net notional volume buy/(sell) of open derivative transactions	0	1,000,000
Power \| MWh
Derivative
Net notional volume buy/(sell) of open derivative transactions	14,000,000	5,000,000
Capacity \| MW/Day
Derivative
Net notional volume buy/(sell) of open derivative transactions	0	(1,000,000)
Interest rate contracts current \| Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	0	0
Derivative liabilities	0	17
Interest rate contracts long-term \| Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	0	0
Derivative liabilities	81	71
Interest rate contracts long-term \| Not Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	0	0
Derivative liabilities	1	0
Commodity contracts current \| Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	203	392
Derivative liabilities	0	2
Commodity contracts current \| Not Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	2,385	1,572
Derivative liabilities	2,089	1,666
Commodity contracts long-term \| Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	63	217
Derivative liabilities	0	0
Commodity contracts long-term \| Not Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	470	541
Derivative liabilities	326	294
Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	266	609
Derivative liabilities	81	90
Not Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	2,855	2,113
Derivative liabilities	$ 2,416	$ 1,960

Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (a),(c)

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624163-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44C

-Subparagraph a

-Clause 1

-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.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 205G

-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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Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

+ References

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Aggregate notional amount of price risk derivatives for instruments with notional amounts expressed in nonmonetary units.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1B

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Notional Amount

-URI http://asc.fasb.org/extlink&oid=6519104

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1A

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5579245-113959

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Aggregate notional amount of interest rate derivatives, which relates to the currency amount specified in the interest rate derivative instruments.

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