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FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Sep. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016
--- | --- | --- | --- | ---
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | $ 6.7 | $ 7.4 | $ 10.6 | $ 5.2
Spire Missouri |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 4.1 | 2.5 | 4.0 | 2.1
Spire Alabama |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.0 | 0.1 | 0.0 | $ 0.0
Quoted Prices in Active Markets (Level 1) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 6.7 | 7.4 | 10.6 |
Short-term debt | 0.0 | 0.0 | 0.0 |
Long-term debt, including current portion | 0.0 | 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 | 4.1 | 2.5 | 4.0 |
Short-term debt | 0.0 | 0.0 | 0.0 |
Long-term debt, including current portion | 0.0 | 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 | 0.0 | 0.1 | 0.0 |
Short-term debt | 0.0 | 0.0 | 0.0 |
Long-term debt, including current portion | 0.0 | 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 | 0.0 |
Short-term debt | 583.6 | 477.3 | 506.4 |
Long-term debt, including current portion | 2,280.1 | 2,210.3 | 2,258.1 |
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 | 0.0 |
Short-term debt | 275.6 | 203.0 | 312.9 |
Long-term debt, including current portion | 1,068.6 | 1,056.9 | 910.7 |
Significant Observable Inputs (Level 2) | Spire Alabama |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.0 | 0.0 | 0.0 |
Short-term debt | 163.1 | 169.9 | 102.5 |
Long-term debt, including current portion | 303.5 | 269.4 | 269.3 |
Carrying Amount |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 6.7 | 7.4 | 10.6 |
Short-term debt | 583.6 | 477.3 | 506.4 |
Long-term debt, including current portion | 2,135.5 | 2,095.0 | 2,071.3 |
Carrying Amount | Spire Missouri |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 4.1 | 2.5 | 4.0 |
Short-term debt | 275.6 | 203.0 | 312.9 |
Long-term debt, including current portion | 974.1 | 973.9 | 804.3 |
Carrying Amount | Spire Alabama |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.0 | 0.1 | 0.0 |
Short-term debt | 163.1 | 169.9 | 102.5 |
Long-term debt, including current portion | 277.8 | 247.8 | 247.7 |
Fair Value |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 6.7 | 7.4 | 10.6 |
Short-term debt | 583.6 | 477.3 | 506.4 |
Long-term debt, including current portion | 2,280.1 | 2,210.3 | 2,258.1 |
Fair Value | Spire Missouri |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 4.1 | 2.5 | 4.0 |
Short-term debt | 275.6 | 203.0 | 312.9 |
Long-term debt, including current portion | 1,068.6 | 1,056.9 | 910.7 |
Fair Value | Spire Alabama |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.0 | 0.1 | 0.0 |
Short-term debt | 163.1 | 169.9 | 102.5 |
Long-term debt, including current portion | $ 303.5 | $ 269.4 | $ 269.3 |

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