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Fair Value Measurements Fair Value Measurements - Investments (Details) number in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Investment Gains (Losses)
Investment fund securities classified as trading and measured at fair value	$ 2,300
Balance in the NDT	1,600
Balance in the ART	$ 519
Period of time where the investor contributes capital to an investment in a private partnership - minimum	3 years
Period of time where the investor contributes capital to an investment in a private partnership - maximum	4 years
Minimum investment period	10 years
Number of readily available quoted exchange prices for the investments	0
SERP
Investment Gains (Losses)
Unrealized gains (losses) on investments	$ 2	$ (4)
LTDCP
Investment Gains (Losses)
Unrealized gains (losses) on investments	1	(2)
NDT
Investment Gains (Losses)
Unrealized gains (losses) on investments	89	(47)
ART
Investment Gains (Losses)
Unrealized gains (losses) on investments	29	$ (17)
Maximum
Investment Gains (Losses)
Equity investments not required to be measured at fair value	1
Equity Funds
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	67
Real Estate Funds
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	$ 5

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Minimum investment period of private partnership investments.

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The fair value of investments held in a trust fund to pay for the costs of decontaminating and decommissioning nuclear facilities.

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Number of readily available quoted exchange prices for the investments

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Fair value portion of arrangements with third parties, including, but not limited to, operating lease arrangement and arrangement in which the entity has agreed to expend funds to procure goods or services, or has agreed to commit resources to supply goods or services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(25))
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The fair value of investments held in a trust fund to pay for the costs of decontaminating and decommissioning facilities, whether such amount is presented as a separate caption or as a parenthetical disclosure on the balance sheet. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-Subparagraph (b)
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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26626-111562

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of investments, not including unrealized gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, held at each balance sheet date and included in earnings for the period.

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