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COMMITMENTS AND CONTINGENCIES - Commitments (Details) $ in Millions | 3 Months Ended
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Dec. 31, 2017 USD ($)
Long-term Purchase Commitment [Line Items] |
Minimum total payments required for natural gas contracts | $ 1,121.2
Spire Missouri |
Long-term Purchase Commitment [Line Items] |
Minimum total payments required for natural gas contracts | 483.8
Spire Alabama |
Long-term Purchase Commitment [Line Items] |
Minimum total payments required for natural gas contracts | $ 263.4

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- Definition The minimum amount the entity agreed to spend under the long-term purchase commitment. + References No definition available. + Details
Name: | us-gaap_LongTermPurchaseCommitmentAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_LongTermPurchaseCommitmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | dei_LegalEntityAxis=lg_SpireMissouriMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | dei_LegalEntityAxis=lg_SpireAlabamaIncMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |