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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, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Spire Alabama | Pension Plans |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average rate of future compensation increase | 3.00% | 3.00% | 2.92%
Expected long-term rate of return on plan assets | 7.25% | 7.50% |
Spire Alabama | Pension Plans | Minimum |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.45% | 4.25% | 4.15%
Expected long-term rate of return on plan assets |  |  | 7.00%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 3.65% | 3.45% |
Spire Alabama | Pension Plans | Maximum |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.50% | 4.30% | 4.25%
Expected long-term rate of return on plan assets |  |  | 7.25%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 3.70% | 3.50% |
Spire Alabama | Postretirement Plans |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.60% | 4.50% | 4.40%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 3.80% | 3.60% |
Spire Alabama | Postretirement Plans | Minimum |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Expected long-term rate of return on plan assets | 4.00% | 4.50% | 4.75%
Spire Alabama | Postretirement Plans | Maximum |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Expected long-term rate of return on plan assets | 6.25% | 7.25% | 7.50%
Spire Missouri | Pension Plans |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.50% | 4.40% | 4.30%
Weighted average rate of future compensation increase | 3.00% | 3.00% | 3.00%
Expected long-term rate of return on plan assets | 7.75% | 7.75% | 7.75%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 3.75% | 3.50% |
Spire Missouri | Postretirement Plans |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.15% | 4.00% | 4.15%
Weighted average rate of future compensation increase | 3.00% | 3.00% | 3.00%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 3.60% | 3.15% |
Weighted average rate of future compensation increase | 3.00% | 3.00% |
Spire Missouri | Postretirement Plans | Minimum |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Expected long-term rate of return on plan assets | 5.75% | 6.00% | 6.25%
Spire Missouri | Postretirement Plans | Maximum |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Expected long-term rate of return on plan assets | 7.75% | 7.75% | 7.75%
Missouri Gas Energy (MGE) | Pension Plans |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.50% | 4.50% | 4.45%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 3.70% | 3.50% |
Missouri Gas Energy (MGE) | Postretirement Plans |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.45% | 4.30% | 4.40%
Expected long-term rate of return on plan assets | 5.50% | 4.75% | 5.00%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 3.60% | 3.45% |
Missouri Gas Energy (MGE) | Postretirement Plans | Minimum |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Expected long-term rate of return on plan assets |  |  | 3.75%
Missouri Gas Energy (MGE) | Postretirement Plans | Maximum |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Expected long-term rate of return on plan assets |  |  | 5.75%
Laclede Gas, Alabama Gas Company, And Missouri Gas Energy | Pension Plans |  |  |
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average rate of future compensation increase | 3.00% | 3.00% |

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