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Risk Management Activities and Derivative Transactions Derivatives Under FTP (Details) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2016 USD ($) | Mar. 31, 2016 USD ($) | Sep. 30, 2015 USD ($)
Natural Gas |  |  |  |
Derivative |  |  |  |
Notional amount |  | 172,000,000 | 172,000,000 | 134,000,000
Fair value |  | $ (7) | $ (7) | $ 1
Number of Price Risk Derivatives Held |  | 32 | 32 | 33
Coal Contract |  |  |  |
Derivative |  |  |  |
Notional amount |  | 25,000,000 | 25,000,000 | 19,000,000
Fair value |  | $ (187) | $ (187) | $ (98)
Number of Price Risk Derivatives Held |  | 12 | 12 | 14
Swap contracts |  |  |  |
Derivative |  |  |  |
Remaining terms |  |  | 2 |
Natural Gas Swap |  |  |  |
Derivative |  |  |  |
Notional amount | [1] | 34,332,500 | 34,332,500 | 51,495,000
Fair value | [1] | $ (87) | $ (87) | $ (116)
Maximum | Natural Gas |  |  |  |
Derivative |  |  |  |
Derivative, Term of Contract |  | 3 years |  |
Maximum | Coal Contract |  |  |  |
Derivative |  |  |  |
Derivative, Term of Contract |  | 3 years |  |
[1] | Fair value amounts presented are based on the net commodity position with the counterparty. Notional amounts disclosed represent the net absolute value of contractual amounts.

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- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13495-108611 + Details
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