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Pension Plans and Other Postretirement Benefits - Amounts Recognized in the Balance Sheets (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
--- | --- | ---
Noncurrent liabilities | $ (74.7) | $ (146.7)
Spire Missouri [Member] |  |
Noncurrent liabilities | (72.4) | (110.0)
Spire Alabama Inc [Member] |  |
Noncurrent liabilities | (2.4) | (28.0)
Pension Plan [Member] |  |
Noncurrent assets | 6.2 | 0.0
Current liabilities | (0.6) | (1.0)
Noncurrent liabilities | (28.1) | (93.9)
Total | (22.5) | (94.9)
Pension Plan [Member] | Spire Missouri [Member] |  |
Noncurrent assets | 3.9 | 0.0
Current liabilities | (0.6) | (1.0)
Noncurrent liabilities | (25.7) | (58.9)
Total | (22.4) | (59.9)
Pension Plan [Member] | Spire Alabama Inc [Member] |  |
Noncurrent assets | 0.0 | 0.0
Current liabilities | 0.0 | 0.0
Noncurrent liabilities | (2.4) | (28.0)
Total | (2.4) | (28.0)
Other Postretirement Benefits Plan [Member] |  |
Noncurrent assets | 288.2 | 248.3
Current liabilities | (0.4) | (0.4)
Noncurrent liabilities | (46.4) | (51.0)
Total | 241.4 | 196.9
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] |  |
Noncurrent assets | 206.1 | 173.9
Current liabilities | (0.4) | (0.4)
Noncurrent liabilities | (46.4) | (51.0)
Total | 159.3 | 122.5
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] |  |
Noncurrent assets | 75.2 | 68.5
Current liabilities | 0.0 | 0.0
Noncurrent liabilities | 0.0 | 0.0
Total | $ 75.2 | $ 68.5

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