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Risk Management Activities and Derivative Transactions Derivatives Under FTP (Details) $ in Millions | 9 Months Ended |
--- | --- | ---
Jun. 30, 2015 USD ($) | Sep. 30, 2014 USD ($)
Derivative |  |  |
FTP transaction limit |  | $ 130 |
Maximum hedge volume |  | 75.00% |
Market value limitation of outstanding construction materials hedging transactions |  | $ 100 |
Portfolio value at risk limit for foreign currency transactions |  | $ 5 |
Swap contracts |  |  |
Derivative |  |  |
Remaining terms |  | 3 years |
Natural Gas Swap |  |  |
Derivative |  |  |
Fair value | [1] | $ (125) | $ (103)
Notional amount | [1] | 63,835,000 | 102,227,500
Coal Contract Derivatives |  |  |
Derivative |  |  |
Fair value |  | $ 0 | $ 0
Futures Contracts |  |  |
Derivative |  |  |
Fair value |  | 0 |
Options Contract |  |  |
Derivative |  |  |
Fair value |  | $ 0 |
[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 Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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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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