Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions	12 Months Ended
Sep. 30, 2015 USD ($) Redemption_options Prices	Sep. 30, 2014 USD ($)
Investment Gains (Losses)
Investment fund securities classified as trading and measured at fair value	$ 2,000
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
NDT unfunded commitments related to private partnerships	$ 89
Number of redemption or limited redemption options \| Redemption_options	0
Number of readily available quoted exchange prices for the investments \| Prices	0
SERP
Investment Gains (Losses)
Unrealized gains (losses) on investments	$ (4)	$ 1
LTDCP
Investment Gains (Losses)
Unrealized gains (losses) on investments	(2)	0
NDT
Investment Gains (Losses)
Unrealized gains (losses) on investments	(47)	35
ART
Investment Gains (Losses)
Unrealized gains (losses) on investments	(17)	$ 15
Maximum
Investment Gains (Losses)
Equity investments not required to be measured at fair value	$ 1

Minimum investment period of private partnership investments.

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Number of readily available quoted exchange prices for the investments

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Number of redemption or limited redemption options related to private partnerships

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Period of time where the investor contributes capital to an investment in a private partnership - maximum

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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.

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-Topic 210

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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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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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