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NOTES PAYABLE AND CREDIT AGREEMENTS - Alagasco's Short-term Borrowings (Details) - USD ($) | 1 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2014 | Sep. 30, 2017 | Sep. 30, 2016
Short-term Debt [Line Items] |  |  |
Weighted average borrowings outstanding |  | $ 485,800,000 | $ 273,900,000
Weighted average interest rate |  | 1.20% | 0.90%
Minimum borrowing outstanding |  | $ 395,500,000 | $ 73,100,000
Maximum borrowing outstanding |  | 675,600,000 | 427,200,000
Short-term debt |  | $ 477,300,000 | $ 398,700,000
Weighted average interest rate |  | 1.50% | 1.10%
Spire Alabama |  |  |
Short-term Debt [Line Items] |  |  |
Weighted average borrowings outstanding |  | $ 106,900,000 | $ 42,600,000
Weighted average interest rate |  | 1.50% | 1.40%
Minimum borrowing outstanding |  | $ 74,000,000 | $ 19,000,000
Maximum borrowing outstanding |  | 171,000,000 | 82,000,000
Short-term debt |  | $ 169,900,000 | $ 82,000,000
Weighted average interest rate |  | 1.50% | 1.50%
Spire Alabama | Line of Credit |  |  |
Short-term Debt [Line Items] |  |  |
Weighted average borrowings outstanding | $ 30,200,000 | $ 28,300,000 | $ 30,200,000
Weighted average interest rate | 1.40% | 1.60% | 1.40%
Minimum borrowing outstanding |  | $ 0 | $ 0
Maximum borrowing outstanding |  | 102,500,000.0 | 82,000,000.0
Short-term debt |  | $ 0 | $ 82,000,000
Weighted average interest rate |  | 0.00% | 1.50%
Spire Alabama | Notes Payable to Related Party |  |  |
Short-term Debt [Line Items] |  |  |
Weighted average borrowings outstanding |  | $ 78,600,000 | $ 12,400,000
Weighted average interest rate |  | 1.40% | 1.40%
Minimum borrowing outstanding |  | $ 0 | $ 0
Maximum borrowing outstanding |  | 171,000,000.0 | 61,900,000.0
Short-term debt |  | $ 169,900,000 | $ 0
Weighted average interest rate |  | 1.50% | 0.00%

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