Restructuring (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2013
Restructuring and Related Activities [Abstract]
Expected increase (decrease) in operation and maintenance expense			$ (500)
Number of positions eliminated					2,000
Severance cost liability	1us-gaap_RestructuringReserve	25us-gaap_RestructuringReserve	1us-gaap_RestructuringReserve	25us-gaap_RestructuringReserve	45us-gaap_RestructuringReserve	4us-gaap_RestructuringReserve	12us-gaap_RestructuringReserve	0us-gaap_RestructuringReserve
Liabilities incurred during the period	0us-gaap_RestructuringCharges	17us-gaap_RestructuringCharges	0us-gaap_RestructuringCharges	29us-gaap_RestructuringCharges
Actual costs paid during the period	(3)us-gaap_PaymentsForRestructuring	(4)us-gaap_PaymentsForRestructuring	(43)us-gaap_PaymentsForRestructuring	(4)us-gaap_PaymentsForRestructuring
Adjustments to estimate during the period	$ 0	$ 0	$ (1)	$ 0

The expected increase (decrease) in operation and maintenance expense.

No definition available.

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- Definition

Amount of cash payments made as the result of exit or disposal activities. Excludes payments associated with a discontinued operation or an asset retirement obligation.

+ References

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- Definition

The number of positions eliminated during the period as a result of restructuring activities.

+ References

No definition available.

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- Definition

Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

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-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 5

-Section P

-Subsection 3, 4

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- Definition

Carrying amount (including both current and noncurrent portions of the accrual) as of the balance sheet date pertaining to a specified type of cost associated with exit from or disposal of business activities or restructuring pursuant to a duly authorized plan.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 420

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SAB TOPIC 5.P.4(b))

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- Definition

Amount of any reversal and other adjustment made during the period to the amount of a previously accrued liability for a specified type of restructuring cost, excluding adjustments for costs incurred during the period, costs settled during the period, and foreign currency translation adjustments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

-Section 50

-Paragraph 1

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