Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Debt securities:
Domestic government	$ 14	$ 14
Corporate credit	8	8
Money market funds	19	18
Investments, Fair Value Disclosure	2	2
Non-qualified benefit plan trust: (2)
Domestic	3	3
Debt securities—domestic government	1	1
Alternative Investments, Fair Value Disclosure	1	1
Assets from price risk management activities: (1) (3)
Electricity	11	7
Natural gas	9	3
Total	68	57
Liabilities from price risk management activities: (1) (3)
Electricity	146	133
Natural gas	157	158
Total	303	291
Fair Value, Inputs, Level 1 [Member]
Debt securities:
Domestic government	5	6
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Domestic	3	3
Debt securities—domestic government	1	1
Assets from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	9	10
Liabilities from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	0	0
Fair Value, Inputs, Level 2 [Member]
Debt securities:
Domestic government	9	8
Corporate credit	8	8
Non-qualified benefit plan trust: (2)
Domestic	0	0
Debt securities—domestic government	0	0
Assets from price risk management activities: (1) (3)
Electricity	11	7
Natural gas	9	3
Total	37	26
Liabilities from price risk management activities: (1) (3)
Electricity	29	28
Natural gas	143	144
Total	172	172
Fair Value, Inputs, Level 3 [Member]
Debt securities:
Domestic government	0	0
Corporate credit	0	0
Non-qualified benefit plan trust: (2)
Domestic	0	0
Debt securities—domestic government	0	0
Assets from price risk management activities: (1) (3)
Electricity	0	0
Natural gas	0	0
Total	0	0
Liabilities from price risk management activities: (1) (3)
Electricity	117	105
Natural gas	14	14
Total	131	119
Estimate of Fair Value Measurement [Member]
Debt securities:
Money market funds	19	18
Investments, Fair Value Disclosure	2	2
Non-qualified benefit plan trust: (2)
Alternative Investments, Fair Value Disclosure	1	1
Assets from price risk management activities: (1) (3)
Total	$ 22	$ 21

Assets from price risk management activities: [Abstract]

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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.

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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 natural gas.

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

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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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Debt securities: [Abstract]

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Liabilities from price risk management activities: [Abstract]

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

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

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Non-qualified benefit plan trust: [Abstract]

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Fair value of investments in certain entities that calculate net asset value per share, including, but not limited to, by unit, membership interest, or other ownership interest. Investments include, but are not limited to, investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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.

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

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
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-Name Accounting Standards Codification
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-Publisher FASB
-Name Accounting Standards Codification
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-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 1
-Subparagraph a, f, g
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Fair value of financial and nonfinancial obligations.

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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.

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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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