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FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2014
--- | --- | --- | --- | ---
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | $ 7.4 | $ 5.2 | $ 13.8 | $ 16.1
Carrying Amount |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 7.4 | 5.2 |  |
Short-term debt | 477.3 | 398.7 |  |
Long-term debt, including current portion | 2,095.0 | 2,070.7 |  |
Fair Value |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 7.4 | 5.2 |  |
Short-term debt | 477.3 | 398.7 |  |
Long-term debt, including current portion | 2,210.3 | 2,257.1 |  |
Spire Missouri |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 2.5 | 2.1 | 1.7 | 3.7
Spire Missouri | Carrying Amount |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 2.5 | 2.1 |  |
Short-term debt | 203.0 | 243.7 |  |
Long-term debt, including current portion | 973.9 | 804.1 |  |
Spire Missouri | Fair Value |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 2.5 | 2.1 |  |
Short-term debt | 203.0 | 243.7 |  |
Long-term debt, including current portion | 1,056.9 | 900.4 |  |
Spire Alabama |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.1 | 0.0 | $ 7.2 | $ 5.6
Spire Alabama | Carrying Amount |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.1 |  |  |
Short-term debt | 169.9 | 82.0 |  |
Long-term debt, including current portion | 247.8 | 247.6 |  |
Spire Alabama | Fair Value |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.1 |  |  |
Short-term debt | 169.9 | 82.0 |  |
Long-term debt, including current portion | 269.4 | 275.5 |  |
Quoted Prices in Active Markets (Level 1) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 7.4 | 5.2 |  |
Short-term debt | 0.0 | 0.0 |  |
Long-term debt, including current portion | 0.0 | 0.0 |  |
Quoted Prices in Active Markets (Level 1) | Spire Missouri |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 2.5 | 2.1 |  |
Short-term debt | 0.0 | 0.0 |  |
Long-term debt, including current portion | 0.0 | 0.0 |  |
Quoted Prices in Active Markets (Level 1) | Spire Alabama |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.1 |  |  |
Short-term debt | 0.0 | 0.0 |  |
Long-term debt, including current portion | 0.0 | 0.0 |  |
Significant Observable Inputs (Level 2) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.0 | 0.0 |  |
Short-term debt | 477.3 | 398.7 |  |
Long-term debt, including current portion | 2,210.3 | 2,257.1 |  |
Significant Observable Inputs (Level 2) | Spire Missouri |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.0 | 0.0 |  |
Short-term debt | 203.0 | 243.7 |  |
Long-term debt, including current portion | 1,056.9 | 900.4 |  |
Significant Observable Inputs (Level 2) | Spire Alabama |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.0 |  |  |
Short-term debt | 169.9 | 82.0 |  |
Long-term debt, including current portion | $ 269.4 | $ 275.5 |  |

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