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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 |

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- Definition 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) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 19 -Article 7 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 17 -Article 9 + Details
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