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NOTES PAYABLE AND CREDIT AGREEMENTS - Narrative (Details) | 12 Months Ended |
--- | --- | ---
Sep. 30, 2017 USD ($) | Sep. 30, 2016 | Dec. 14, 2016 bank
Line of Credit Facility [Line Items] |  |  |
Long-term Line of Credit | $ 477,300,000 |  |
Speculative interest rate increase percent | 1.00% |  |
Decrease in cash flow with an impact of 100 basis point interest rate increase | $ 4,900,000 |  |
Spire Missouri |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term Line of Credit | 203,000,000 |  |
Long-term debt | $ 975,000,000 |  |
Speculative interest rate increase percent | 1.00% |  |
Decrease in cash flow with an impact of 100 basis point interest rate increase | $ 2,800,000 |  |
Spire Alabama |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term Line of Credit | 169,900,000 |  |
Long-term debt | $ 250,000,000 |  |
Speculative interest rate increase percent | 1.00% |  |
Decrease in cash flow with an impact of 100 basis point interest rate increase | $ 1,100,000 |  |
Spire Energy South Inc |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term Line of Credit | 12,900,000 |  |
Spire STL Pipeline LLC |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term Line of Credit | 26,600,000 |  |
Line of Credit |  |  |
Line of Credit Facility [Line Items] |  |  |
Line of credit facility borrowing capacity | $ 975,000,000.0 |  |
Minimum required ratio of earnings before interest, taxes, depreciation, and amortization (EBITDA) times interest expense as required by the line of credit covenant | 70.00% |  |
Debt to total capitalization ratio | 0.56 |  |
Line of Credit | Spire Missouri |  |  |
Line of Credit Facility [Line Items] |  |  |
Line of credit facility borrowing capacity | $ 475,000,000.0 |  |
Debt to total capitalization ratio | 0.50 |  |
Line of Credit | Spire Alabama |  |  |
Line of Credit Facility [Line Items] |  |  |
Line of credit facility borrowing capacity | $ 200,000,000.0 |  |
Debt to total capitalization ratio | 0.33 |  |
Line of Credit | Liabilities | Lender Concentration Risk |  |  |
Line of Credit Facility [Line Items] |  |  |
Line Of Credit Facility, Number Of Banks In Agreement | bank |  |  | 11
Percent of debt by a singe bank |  | 12.30% |
Subject to Callable at Par | Spire Missouri |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term debt | $ 520,000,000 |  |
Spire | Line of Credit |  |  |
Line of Credit Facility [Line Items] |  |  |
Line of credit facility borrowing capacity | 300,000,000.0 |  |
Other Affiliates |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term Line of Credit | 27,600,000 |  |
Other subsidiaries |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term Line of Credit | $ 440,000,000 |  |

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