Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Estimate of Fair Value, Fair Value Disclosure [Member]
Nuclear decommissioning trust (1):
Money market funds	$ 14	$ 15
Debt securities:
Domestic government	16	15
Corporate credit	8	8
Non-qualified benefit plan trust (2):
Fair Value - Money market funds		2
Equity securities:
Domestic	6	4
International		1
Debt securities - domestic government	2	2
Assets from price risk management activities:
Electricity	5	1
Natural gas	7	5
Total	58	53
Liabilities from price risk management activities: (1) (3)
Electricity	78	82
Natural gas	91	118
Total	169	200
Fair Value, Inputs, Level 1 [Member]
Nuclear decommissioning trust (1):
Money market funds	0	0
Debt securities:
Domestic government	9	7
Corporate credit	0	0
Non-qualified benefit plan trust (2):
Fair Value - Money market funds		0
Equity securities:
Domestic	3	2
International		1
Debt securities - domestic government	2	2
Assets from price risk management activities:
Electricity	0	0
Natural gas	0	0
Total	14	12
Liabilities from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	0	0
Fair Value, Inputs, Level 2 [Member]
Nuclear decommissioning trust (1):
Money market funds	14	15
Debt securities:
Domestic government	7	8
Corporate credit	8	8
Non-qualified benefit plan trust (2):
Fair Value - Money market funds		2
Equity securities:
Domestic	3	2
International		0
Debt securities - domestic government	0	0
Assets from price risk management activities:
Electricity	5	1
Natural gas	6	3
Total	43	39
Liabilities from price risk management activities: (1) (3)
Electricity	42	72
Natural gas	81	110
Total	123	182
Fair Value, Inputs, Level 3 [Member]
Nuclear decommissioning trust (1):
Money market funds	0	0
Debt securities:
Domestic government	0	0
Corporate credit	0	0
Non-qualified benefit plan trust (2):
Fair Value - Money market funds		0
Equity securities:
Domestic	0	0
International		0
Debt securities - domestic government	0	0
Assets from price risk management activities:
Electricity	0	0
Natural gas	1	2
Total	1	2
Liabilities from price risk management activities: (1) (3)
Electricity	36	10
Natural gas	10	8
Total	$ 46	$ 18

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Carrying amount as of the balance sheet date of the asset arising from commodity contracts related to purchases or sales of electricity.

+ References

No definition available.

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- Definition

Carrying amount as of the balance sheet date of the asset arising from commodity contracts related to purchases or sales of natural gas.

+ References

No definition available.

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- Definition

Carrying amount as of the balance sheet date of the liabilities arising from commodity contracts related to purchases or sales of electricity.

+ References

No definition available.

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- Definition

Carrying amount as of the balance sheet date of the liabilities arising from commodity contracts related to purchases or sales of natural gas.

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No definition available.

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- Definition

Common stock asset securities held in Non-qualified benefit plan trust, which are carried at fair value

+ References

No definition available.

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- Definition

Fair Value - Money market funds

+ References

No definition available.

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- Definition

Debt mutual funds assets in the Non-qualified benefit plan trust, which are carried at fair value.

+ References

No definition available.

+ Details

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- Definition

Equity mutual fund assets held in the Non-qualified benefit plan trust, which are carried at fair value.

+ References

No definition available.

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- Definition

The aggregate fair value as of the balance sheet date of financial instruments and other positions owned by the entity including: (1) mortgages, mortgage-backed and asset backed securities; (2) US government and agency obligations; (3) state and municipal government obligations; (4) other sovereign government debt; (5) corporate obligations; (6) corporate equities; (7) principal investments; (8) derivative contracts; and (9) physical commodities. Includes both pledged and unpledged holdings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 940

-SubTopic 320

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6475986&loc=d3e42546-110969

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

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

-Paragraph 38, 40

-IssueDate 2006-05-01

-Chapter 4

-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 AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

-Paragraph 80

-Subparagraph Exhibit 4-3

-IssueDate 2006-05-01

-Chapter 4

-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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- Definition

The fair value as of the balance sheet date of firm holdings in fixed income or other debt securities issued by a corporation or other issuer not elsewhere specified in the taxonomy. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

+ References

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- Definition

This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

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- Definition

Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

-URI http://asc.fasb.org/extlink&oid=6507016

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 8, 9

-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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- Definition

Debt (bills, notes or bonds) that are issued by the government of the United States which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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