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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Assumptions Used to Calculate Net Periodic Cost and Benefit Obligations (Details) | 12 Months Ended
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Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2012
Pension Plans |  |  |  |  |  |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average rate of future compensation increase (all plans) | 3.00% |  | 3.00% |  |  |
Postretirement Plans |  |  |  |  |  |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average rate of future compensation increase (all plans) | 3.00% |  | 3.00% |  |  |
Alagasco | Pension Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average rate of future compensation increase | 2.92% |  |  |  |  |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate |  |  | 0.00% |  |  |
Alagasco | Postretirement Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.25% |  |  |  |  |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.40% |  | 0.00% |  |  |
Laclede Gas | Pension Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.70% |  | 3.95% |  | 5.10% |
Weighted average rate of future compensation increase | 3.00% | [1] | 3.00% | [1] | 3.00% | [1]
Expected long-term rate of return on plan assets | 7.75% | [1] | 7.75% | [1] | 7.75% | [1]
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.30% |  | 4.70% |  |  |
Laclede Gas | Postretirement Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.60% |  | 3.80% |  | 5.05% |
Weighted average rate of future compensation increase | 3.00% | [1] | 3.00% | [1] | 3.00% | [1]
Expected long-term rate of return on plan assets |  |  | 7.75% |  | 7.75% |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.15% |  | 4.60% |  |  |
Missouri Gas Energy (MGE) | Pension Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average discount rate | 5.00% |  | 5.05% |  | 0.00% |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.45% |  | 5.00% |  |  |
Missouri Gas Energy (MGE) | Postretirement Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.95% |  | 5.00% |  | 0.00% |
Expected long-term rate of return on plan assets |  |  | 5.75% |  | 0.00% |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.40% |  | 4.95% |  |  |
Minimum | Alagasco | Pension Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.00% |  |  |  |  |
Weighted average rate of future compensation increase | 7.00% |  |  |  |  |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.15% |  |  |  |  |
Minimum | Alagasco | Postretirement Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Expected long-term rate of return on plan assets | 4.75% |  |  |  |  |
Minimum | Laclede Gas | Postretirement Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Expected long-term rate of return on plan assets | 6.25% |  |  |  |  |
Minimum | Missouri Gas Energy (MGE) | Postretirement Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Expected long-term rate of return on plan assets | 3.75% |  |  |  |  |
Maximum | Alagasco | Pension Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.05% |  |  |  |  |
Weighted average rate of future compensation increase | 7.25% |  |  |  |  |
Assumptions used to calculate benefit obligations [Abstract] |  |  |  |  |  |
Weighted average discount rate | 4.25% |  |  |  |  |
Maximum | Alagasco | Postretirement Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Expected long-term rate of return on plan assets | 7.50% |  |  |  |  |
Maximum | Laclede Gas | Postretirement Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Expected long-term rate of return on plan assets | 7.75% |  |  |  |  |
Maximum | Missouri Gas Energy (MGE) | Postretirement Plans |  |  |  |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |  |  |  |
Expected long-term rate of return on plan assets | 5.75% |  |  |  |  |
[1] | Assumptions for weighted average rate of future compensation increase and expected long-term rate of return on plan assets are the same for both Laclede Gas and MGE plans.

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- Definition The interest rate used to adjust for the time value of money for the plan. + 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 (k)(1) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + 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 (k)(2) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition The interest rate used to adjust for the time value of money. + 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 (k)(1) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512013 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + 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 (k)(2) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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