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Leases - Schedule of Balance Sheet and Weighted Average Information About Operating Leases (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
--- | --- | ---
Right-of-use assets | $ 59.2 | $ 63.1
Lease liabilities, current | 6.1 | 6.6
Lease liabilities, noncurrent | $ 59.1 | $ 62.7
Weighted-average remaining lease term (in years) | 12 years 7 months 6 days | 13 years 6 months
Weighted-average discount rate | 4.20% | 4.20%
Spire Missouri [Member] |  |
Right-of-use assets | $ 1.6 | $ 1.7
Lease liabilities, current | 0.5 | 0.5
Lease liabilities, noncurrent | $ 1.2 | $ 1.3
Weighted-average remaining lease term (in years) | 4 years 9 months 18 days | 5 years 1 month 6 days
Weighted-average discount rate | 3.80% | 3.40%
Spire Alabama Inc [Member] |  |
Right-of-use assets | $ 17.0 | $ 18.1
Lease liabilities, current | 2.0 | 1.9
Lease liabilities, noncurrent | $ 20.9 | $ 22.3
Weighted-average remaining lease term (in years) | 11 years 3 months 18 days | 12 years 3 months 18 days
Weighted-average discount rate | 3.70% | 3.70%

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