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FAIR VALUE OF FINANCIAL INSTRUMENTS (Tables) | 12 Months Ended
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Sep. 30, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |
Fair value of financial instruments | The carrying amounts and estimated fair values of financial instruments not measured at fair value on a recurring basis for the Company are as follows:
|  |  |  |  | Classification of Estimated Fair Value
| Carrying Amount |  |  | Fair Value |  |  | Quoted Prices in Active Markets (Level 1) |  |  | Significant Observable Inputs (Level 2) |
As of September 30, 2017 |  |  |  |  |  |  |
Cash and cash equivalents | $ | 7.4 |  |  | $ | 7.4 |  |  | $ | 7.4 |  |  | $ | — |
Short-term debt | 477.3 |  |  | 477.3 |  |  | — |  |  | 477.3 |
Long-term debt, including current portion | 2,095.0 |  |  | 2,210.3 |  |  | — |  |  | 2,210.3 |
As of September 30, 2016 |  |  |  |  |  |  |
Cash and cash equivalents | $ | 5.2 |  |  | $ | 5.2 |  |  | $ | 5.2 |  |  | $ | — |
Short-term debt | 398.7 |  |  | 398.7 |  |  | — |  |  | 398.7 |
Long-term debt, including current portion | 2,070.7 |  |  | 2,257.1 |  |  | — |  |  | 2,257.1 |
Spire Missouri |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |
Fair value of financial instruments | The carrying amounts and estimated fair values of financial instruments not measured at fair value on a recurring basis for Spire Missouri are as follows:
|  |  |  |  | Classification of Estimated Fair Value
| Carrying Amount |  |  | Fair Value |  |  | Quoted Prices in Active Markets (Level 1) |  |  | Significant Observable Inputs (Level 2) |
As of September 30, 2017 |  |  |  |  |  |  |
Cash and cash equivalents | $ | 2.5 |  |  | $ | 2.5 |  |  | $ | 2.5 |  |  | $ | — |
Short-term debt | 203.0 |  |  | 203.0 |  |  | — |  |  | 203.0 |
Long-term debt, including current portion | 973.9 |  |  | 1,056.9 |  |  | — |  |  | 1,056.9 |
As of September 30, 2016 |  |  |  |  |  |  |
Cash and cash equivalents | $ | 2.1 |  |  | $ | 2.1 |  |  | $ | 2.1 |  |  | $ | — |
Short-term debt | 243.7 |  |  | 243.7 |  |  | — |  |  | 243.7 |
Long-term debt | 804.1 |  |  | 900.4 |  |  | — |  |  | 900.4 |
Spire Alabama |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |
Fair value of financial instruments | The carrying amounts and estimated fair values of financial instruments not measured at fair value on a recurring basis for Spire Alabama are as follows:
|  |  |  |  | Classification of Estimated Fair Value
| Carrying Amount |  |  | Fair Value |  |  | Quoted Prices in Active Markets (Level 1) |  |  | Significant Observable Inputs (Level 2) |
As of September 30, 2017 |  |  |  |  |  |  |
Cash and cash equivalents | $ | 0.1 |  |  | $ | 0.1 |  |  | $ | 0.1 |  |  | $ | — |
Short-term debt | 169.9 |  |  | 169.9 |  |  | — |  |  | 169.9 |
Long-term debt | 247.8 |  |  | 269.4 |  |  | — |  |  | 269.4 |
As of September 30, 2016 |  |  |  |  |  |  |
Short-term debt | $ | 82.0 |  |  | $ | 82.0 |  |  | $ | — |  |  | $ | 82.0 |
Long-term debt, including current portion | 247.6 |  |  | 275.5 |  |  | — |  |  | 275.5 |

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