Commitments and Contingencies - Additional Information (Details) $ in Millions |
9 Months Ended |
|---|---|
|
Jun. 30, 2025
USD ($)
| |
| Long-Term Purchase Commitment [Line Items] | |
| Long-term purchase commitment, amount | $ 1,789.4 |
| Capital committed to contribute to partnership | 25.3 |
| Remaining unfunded capital commitment | 15.5 |
| Spire Missouri [Member] | |
| Long-Term Purchase Commitment [Line Items] | |
| Long-term purchase commitment, amount | 1,451.3 |
| Spire Alabama Inc [Member] | |
| Long-Term Purchase Commitment [Line Items] | |
| Long-term purchase commitment, amount | $ 519.2 |