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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, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Pension Plans |  |  |
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Discount rates | 3.00% | 3.00% |
Pension Plans | Spire Missouri East |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.20% | 4.30% | 3.75%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 2.85% | 3.20% |
Pension Plans | Spire Missouri West |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.15% | 4.35% | 3.70%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 2.75% | 3.15% |
Pension Plans | Spire Missouri |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average rate of future compensation increase | 3.00% | 3.00% | 3.00%
Expected long-term rate of return on plan assets | 7.25% | 7.50% | 7.75%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Discount rates | 2.85% | 3.20% |
Expected long-term rate of return on plan assets | 7.25% | 7.50% | 7.75%
Pension Plans | Spire Alabama |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate |  | 4.35% |
Weighted average rate of future compensation increase | 3.00% | 3.00% | 3.00%
Expected long-term rate of return on plan assets | 7.25% | 7.25% | 7.25%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Discount rates | 2.95% | 3.20% |
Expected long-term rate of return on plan assets | 7.25% | 7.25% | 7.25%
Pension Plans | Spire Alabama | Maximum |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.25% |  | 3.70%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 2.95% | 3.25% |
Pension Plans | Spire Alabama | Minimum [Member] |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.20% |  | 3.65%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 2.80% | 3.20% |
Other Postretirement Plans | Spire Missouri East |  |  |
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Discount rates | 3.00% | 3.00% |
Other Postretirement Plans | Spire Missouri |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.15% | 4.30% | 3.60%
Weighted average rate of future compensation increase | 3.00% | 3.00% | 3.00%
Expected long-term rate of return on plan assets | 6.25% | 6.25% |
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 2.75% | 3.15% |
Expected long-term rate of return on plan assets | 6.25% | 6.25% |
Other Postretirement Plans | Spire Missouri | Maximum |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Expected long-term rate of return on plan assets |  |  | 7.75%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Expected long-term rate of return on plan assets |  |  | 7.75%
Other Postretirement Plans | Spire Missouri | Minimum [Member] |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Expected long-term rate of return on plan assets |  |  | 5.75%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Expected long-term rate of return on plan assets |  |  | 5.75%
Other Postretirement Plans | Spire Alabama |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Weighted average discount rate | 3.15% | 4.30% | 3.80%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Weighted average discount rate | 2.75% | 3.15% |
Other Postretirement Plans | Spire Alabama | Maximum |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Expected long-term rate of return on plan assets | 6.25% | 6.25% | 6.00%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Expected long-term rate of return on plan assets | 6.25% | 6.25% | 6.00%
Other Postretirement Plans | Spire Alabama | Minimum [Member] |  |  |
Assumptions used to calculate net periodic cost [Abstract] |  |  |
Expected long-term rate of return on plan assets | 5.00% | 5.00% | 3.75%
Assumptions used to calculate benefit obligations [Abstract] |  |  |
Expected long-term rate of return on plan assets | 5.00% | 5.00% | 3.75%

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