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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Fair Value Measurements of Plan Assets (Details) (USD $) In Millions, unless otherwise specified | Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2012
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Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | $ 9.3 |  | $ 0 |  |
Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 9.3 |  | 0 |  |
Pension Plans |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 506.6 | [1] | 345.4 | [1] | 274.1
Pension Plans | Cash and cash equivalents |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 10.2 |  | 18.2 |  |
Pension Plans | Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 138.2 |  | 115.8 |  |
Pension Plans | US bond mutual funds |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 73.6 |  | 17.7 |  |
Pension Plans | US government |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 64.5 |  | 55.7 |  |
Pension Plans | US corporate |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 164.0 |  | 110.9 |  |
Pension Plans | US municipal |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 8.2 |  | 6.8 |  |
Pension Plans | International |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 35.5 |  | 21.6 |  |
Pension Plans | Alternative |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 13.4 |  |  |  |
Pension Plans | Derivative instruments |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | (1.0) | [2] | (1.3) | [3] |
Pension Plans | Quoted Prices in Active Markets (Level 1) |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 136.4 |  | 35.9 |  |
Pension Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 8.6 |  | 18.2 |  |
Pension Plans | Quoted Prices in Active Markets (Level 1) | Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 54.2 |  |  |  |
Pension Plans | Quoted Prices in Active Markets (Level 1) | US bond mutual funds |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 73.6 |  | 17.7 |  |
Pension Plans | Significant Observable Inputs (Level 2) |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 360.9 |  | 309.5 |  |
Pension Plans | Significant Observable Inputs (Level 2) | Cash and cash equivalents |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 1.6 |  |  |  |
Pension Plans | Significant Observable Inputs (Level 2) | Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 74.7 |  | 115.8 |  |
Pension Plans | Significant Observable Inputs (Level 2) | US bond mutual funds |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets |  |  | 0 |  |
Pension Plans | Significant Observable Inputs (Level 2) | US government |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 64.5 |  | 55.7 |  |
Pension Plans | Significant Observable Inputs (Level 2) | US corporate |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 164.0 |  | 110.9 |  |
Pension Plans | Significant Observable Inputs (Level 2) | US municipal |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 8.2 |  | 6.8 |  |
Pension Plans | Significant Observable Inputs (Level 2) | International |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 35.5 |  | 21.6 |  |
Pension Plans | Significant Observable Inputs (Level 2) | Alternative |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 13.4 |  |  |  |
Pension Plans | Significant Observable Inputs (Level 2) | Derivative instruments |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | (1.0) | [2] | (1.3) | [3] |
Pension Plans | Significant Unobservable Inputs (Level 3) |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 9.3 |  |  |  |
Pension Plans | Significant Unobservable Inputs (Level 3) | Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 9.3 |  |  |  |
Postretirement Plans |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 222.5 | [1] | 111.6 | [1] | 67.4
Postretirement Plans | Cash and cash equivalents |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 2.3 |  | 1.4 |  |
Postretirement Plans | Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 213.0 |  | 110.2 |  |
Postretirement Plans | International |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 7.2 |  |  |  |
Postretirement Plans | Quoted Prices in Active Markets (Level 1) |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 222.5 |  | 111.6 |  |
Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 2.3 |  | 1.4 |  |
Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 213.0 |  | 110.2 |  |
Postretirement Plans | Quoted Prices in Active Markets (Level 1) | International |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 7.2 |  |  |  |
Laclede Gas | Pension Plans |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 387.4 |  | 345.4 |  | 274.1
Derivative Assets, fair value measurements | 2.9 |  | 4.2 |  |
Cash margin payable | 3.9 |  | 5.5 |  |
Laclede Gas | Pension Plans | Cash and cash equivalents |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 8.3 |  | 18.2 |  |
Laclede Gas | Pension Plans | Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 48.5 |  | 115.8 |  |
Laclede Gas | Pension Plans | US bond mutual funds |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 73.6 |  | 17.7 |  |
Laclede Gas | Pension Plans | US government |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 60.5 |  | 55.7 |  |
Laclede Gas | Pension Plans | US corporate |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 154.5 |  | 110.9 |  |
Laclede Gas | Pension Plans | US municipal |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 8.2 |  | 6.8 |  |
Laclede Gas | Pension Plans | International |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 34.8 |  | 21.6 |  |
Laclede Gas | Pension Plans | Derivative instruments |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | (1.0) | [2] | (1.3) | [3] |
Laclede Gas | Pension Plans | Quoted Prices in Active Markets (Level 1) |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 81.9 |  | 35.9 |  |
Laclede Gas | Pension Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 8.3 |  | 18.2 |  |
Laclede Gas | Pension Plans | Quoted Prices in Active Markets (Level 1) | US bond mutual funds |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 73.6 |  | 17.7 |  |
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 296.2 |  | 309.5 |  |
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 39.2 |  | 115.8 |  |
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | US bond mutual funds |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets |  |  | 0 |  |
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | US government |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 60.5 |  | 55.7 |  |
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | US corporate |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 154.5 |  | 110.9 |  |
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | US municipal |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 8.2 |  | 6.8 |  |
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | International |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 34.8 |  | 21.6 |  |
Laclede Gas | Pension Plans | Significant Observable Inputs (Level 2) | Derivative instruments |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | (1.0) | [2] | (1.3) | [3] |
Laclede Gas | Pension Plans | Significant Unobservable Inputs (Level 3) |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 9.3 |  |  |  |
Laclede Gas | Pension Plans | Significant Unobservable Inputs (Level 3) | Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 9.3 |  |  |  |
Laclede Gas | Postretirement Plans |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 137.2 |  | 111.6 |  | 67.4
Laclede Gas | Postretirement Plans | Cash and cash equivalents |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 2.3 |  | 1.4 |  |
Laclede Gas | Postretirement Plans | Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 134.9 |  | 110.2 |  |
Laclede Gas | Postretirement Plans | Quoted Prices in Active Markets (Level 1) |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 137.2 |  | 111.6 |  |
Laclede Gas | Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Cash and cash equivalents |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | 2.3 |  | 1.4 |  |
Laclede Gas | Postretirement Plans | Quoted Prices in Active Markets (Level 1) | Stock/Bond mutual fund |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Fair value measurements of plan assets | $ 134.9 |  | $ 110.2 |  |
[1] | Includes Alagasco.
[2] | Derivative assets of $2.9 net of cash margin payable of $3.9.
[3] | Derivative assets of $4.2 net of cash margin payable of $5.5.

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- Definition Amount of cash received as collateral related to open derivative positions. + References No definition available. + Details
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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=29635418&loc=d3e12355-114930 + 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 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13433-108611 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 15 -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13495-108611 Reference 4: 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 + Details
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