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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) | 12 Months Ended
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Sep. 30, 2017 USD ($) operating_segment change | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($)
Related Party Transaction [Line Items] |  |  |
Number of operating segments | operating_segment | 2 |  |
Utility Plant, Depreciation and Amortization [Abstract] |  |  |
Accruals for capital expenditures | $ 41,000,000 | $ 30,400,000 | $ 13,400,000
Amount by which the replacement cost exceeds (less than) the cost basis value of stored gas inventories | $ 20,800,000 | 11,400,000 |
Number of months of the original maturity of debt instruments purchased | 3 months |  |
Finance receivable, net | $ 12,500,000 | 11,800,000 |
Retrospective Effect is ASU had been Adopted | ASU 2015-03 | New Accounting Pronouncement, Early Adoption, Effect | Long-Term Debt |  |  |
Utility Plant, Depreciation and Amortization [Abstract] |  |  |
Deferred finance costs | $ 13,200,000 | $ 13,000,000 |
Spire Missouri |  |  |
Utility Plant, Depreciation and Amortization [Abstract] |  |  |
Average annual depreciation and amortization expense percentage | 3.00% | 3.00% | 3.00%
Accruals for capital expenditures | $ 28,900,000 | $ 14,800,000 | $ 9,600,000
Number of PGA discretionary changes | change | 3 |  |
Months between PGA discretionary changes | 2 months |  |
Cost savings retention percentage | 10.00% |  |
Maximum amount of cost savings allowable | $ 3,000,000.0 |  |
Accrued unbilled revenues | 30,100,000 | 26,100,000 |
Spire Missouri | Retrospective Effect is ASU had been Adopted | ASU 2015-03 | New Accounting Pronouncement, Early Adoption, Effect | Long-Term Debt |  |  |
Utility Plant, Depreciation and Amortization [Abstract] |  |  |
Deferred finance costs | 4,200,000 | 4,800,000 |
Spire Alabama |  |  |
Utility Plant, Depreciation and Amortization [Abstract] |  |  |
Accruals for capital expenditures | $ 9,400,000 | 6,800,000 | $ 3,100,000
Composite depreciation rate | 3.10% |  |
Accrued unbilled revenues | $ 0 | 5,900,000 |
Threshold period past due for collection | 90 days |  |
Threshold period past due for write-off of financing receivable | 12 months |  |
Write off on an allowance for credit losses | $ 400,000 | 400,000 |
Spire Alabama | Retrospective Effect is ASU had been Adopted | ASU 2015-03 | New Accounting Pronouncement, Early Adoption, Effect | Long-Term Debt |  |  |
Utility Plant, Depreciation and Amortization [Abstract] |  |  |
Deferred finance costs | 2,600,000 | 2,400,000 |
Spire Alabama | Financing Receivables, Equal to Greater than 90 Days Past Due |  |  |
Utility Plant, Depreciation and Amortization [Abstract] |  |  |
Finance receivable 90 days past due | $ 400,000 | $ 400,000 |

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