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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) | Mar. 31, 2017 USD ($) | Mar. 31, 2018 USD ($) | Mar. 31, 2017 USD ($)
Investment Gains (Losses) |  |  |  |
Investment fund securities classified as trading and measured at fair value | $ 2,700 |  | $ 2,700 |
Balance in NDT | 1,900 |  | 1,900 |
Balance in ART | $ 648 |  | $ 648 |
Period of time where the investor contributes capital to an investment in a private partnership - minimum |  |  | three |
Period of time where the investor contributes capital to an investment in a private partnership - maximum |  |  | four |
Minimum investment period |  |  | 10 years |
Number of readily available quoted exchange prices for the investments | 0 |  | 0 |
SERP |  |  |  |
Investment Gains (Losses) |  |  |  |
Unrealized gains (losses) on investments | $ (2) | $ 2 | $ 0 | $ 1
DCP |  |  |  |
Investment Gains (Losses) |  |  |  |
Unrealized gains (losses) on investments | (1) | 1 | 0 | 0
NDT |  |  |  |
Investment Gains (Losses) |  |  |  |
Unrealized gains (losses) on investments | (75) | 42 | (29) | 35
ART |  |  |  |
Investment Gains (Losses) |  |  |  |
Unrealized gains (losses) on investments | (14) | $ 19 | 6 | $ 22
Equity Funds |  |  |  |
Investment Gains (Losses) |  |  |  |
NDT unfunded commitments related to private equity and real estate | 73 |  | 73 |
Real Estate Funds |  |  |  |
Investment Gains (Losses) |  |  |  |
NDT unfunded commitments related to private equity and real estate | 5 |  | 5 |
Private Equity Funds [Member] |  |  |  |
Investment Gains (Losses) |  |  |  |
NDT unfunded commitments related to private equity and real estate | $ 14 |  | $ 14 |

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- Definition The fair value of investments held in a trust fund to pay for the costs of decontaminating and decommissioning nuclear facilities. + References No definition available. + Details
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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 + Details
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- Definition Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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