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COMMITMENTS AND CONTINGENCIES - Commitments (Details) $ in Millions | 12 Months Ended
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Sep. 30, 2017 USD ($)
Long-term Purchase Commitment [Line Items] |
Minimum total payments required for natural gas contracts | $ 1,281.8
Spire Missouri |
Long-term Purchase Commitment [Line Items] |
Minimum total payments required for natural gas contracts | 563.9
Spire Missouri | Purchase Commitment | Subsidiary of Common Parent |
Long-term Purchase Commitment [Line Items] |
Annual purchase commitment | $ 1.0
Commitment termination minimum required notification period | 6 months
Spire Alabama |
Long-term Purchase Commitment [Line Items] |
Minimum total payments required for natural gas contracts | $ 285.6

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