Fair Value Measurements - Level 3 (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commodity [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 60	$ 32	$ 23
Changes included in earnings	19	34	10
Changes in fair value recognized in other comprehensive income	0	(2)	0
Changes in fair value recognized in net regulatory assets	5	1	(2)
Purchases	1	4	27
Sales	0	0	0
Settlements	1	(9)	(26)
Transfers from level 2	(35)	0	0
Ending Balance	51	60	32
Auction rate securities [Member]
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	44	41	35
Changes included in earnings	0	0	0
Changes in fair value recognized in other comprehensive income	1	3	7
Changes in fair value recognized in net regulatory assets	0	0	0
Purchases	0	0	0
Sales	0	0	(1)
Settlements	0	0	0
Transfers from level 2	0	0	0
Ending Balance	$ 45	$ 44	$ 41

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

+ References

No definition available.

+ Details

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- Definition

Amount of gain (loss) recognized in regulatory assets or liabilities, net, arising from assets, liabilities and financial instruments classified in shareholders' equity measured at fair value on a recurring basis using unobservable inputs (level 3).

+ References

No definition available.

+ Details

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- Definition

This element represents transfers in to assets and liabilities, net, measured at fair value on a recurring basis using unobservable inputs (Level 3) from Level 2, which have taken place during the period.

+ References

No definition available.

+ Details

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- Definition

This element represents assets and liabilities, net measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

No definition available.

+ Details

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- Definition

This item represents the amount of the total realized and unrealized gains or losses for the period which are included in the statement of income (or changes in net assets); the fair value of which assets and liabilities was or is measured on a recurring basis using significant unobservable inputs (Level 3).

+ 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 (c)(1)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ 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 (c)(2)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

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- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ 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 (c)(2)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

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- Definition

Amount of gain (loss) recognized in other comprehensive income, arising from assets, liabilities and financial instruments classified in shareholders' equity measured at fair value on a recurring basis using unobservable inputs (level 3).

+ 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 (c)(1a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

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- Details

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- Details

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