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REGULATORY MATTERS - Narrative (Details) - USD ($) |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 Months Ended | 12 Months Ended |  |  |  |  |  |
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Sep. 30, 2017 | Jul. 31, 2017 | Apr. 30, 2017 | Apr. 11, 2017 | Apr. 04, 2017 | Feb. 03, 2017 | Jan. 31, 2017 | Jan. 18, 2017 | Nov. 30, 2016 | Nov. 29, 2016 | Sep. 30, 2016 | Jul. 31, 2016 | Jan. 01, 2014 | Sep. 30, 2017 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015 | Mar. 31, 2018 | Sep. 18, 2017 | May 30, 2017 | Apr. 15, 2016 | Feb. 10, 2016 | Dec. 01, 2015
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Remaining Recovery Period of Certain Regulatory Assets for which No Return on Investment During Recovery Period is Provided |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20 years |  |  |  |  |  |  |  |
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15 years |  |  |  |  |  |  |  |
Total Current Regulatory Liabilities | $ 22,600,000 |  |  |  |  |  |  |  |  |  | $ 30,600,000 |  |  | $ 22,600,000 | $ 22,600,000 | $ 30,600,000 |  |  |  |  |  |  |
Regulatory Liabilities | 179,800,000 |  |  |  |  |  |  |  |  |  | 161,300,000 |  |  | 179,800,000 | 179,800,000 | 161,300,000 |  |  |  |  |  |  |
Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory preferred stock and debt securities issuance, amount requested |  |  |  | $ 25,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 550,000,000 |  |
Regulatory preferred stock and debt securities issuance, amount authorized | 300,000,000 |  |  |  |  |  |  |  |  |  |  |  |  | 300,000,000 | 300,000,000 |  |  |  |  | $ 300,000,000.0 |  | $ 300,000,000.0 |
Approved return rate on equity |  |  |  | 10.35% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Current Regulatory Liabilities | 2,700,000 |  |  |  |  |  |  |  |  |  | 1,300,000 |  |  | 2,700,000 | 2,700,000 | 1,300,000 |  |  |  |  |  |  |
Regulatory Liabilities | 83,900,000 |  |  |  |  |  |  |  |  |  | 68,600,000 |  |  | 83,900,000 | 83,900,000 | 68,600,000 |  |  |  |  |  |  |
Missouri Gas Energy (MGE) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory preferred stock and debt securities issuance, amount requested |  |  |  | $ 34,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved return rate on equity |  |  |  | 10.35% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory preferred stock and debt securities issuance, amount authorized | 75,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  | 75,000,000.0 | 75,000,000.0 |  |  |  |  |  |  |  |
Total Current Regulatory Liabilities | 12,000,000 |  |  |  |  |  |  |  |  |  | 22,700,000 |  |  | 12,000,000 | 12,000,000 | 22,700,000 |  |  |  |  |  |  |
Regulatory Liabilities | 51,600,000 |  |  |  |  |  |  |  |  |  | 64,400,000 |  |  | 51,600,000 | $ 51,600,000 | 64,400,000 |  |  |  |  |  |  |
Potential Performance-Based Adjustment, Percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.05% |  |  |  |  |  |  |  |
Spire Alabama | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit facility borrowing capacity | 200,000,000 |  |  |  |  |  |  |  |  |  |  |  |  | 200,000,000 | $ 200,000,000 |  |  |  |  |  |  |  |
Line of Credit Facility, Maximum Amount Outstanding During Period |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 25,000,000 |  |  |  |  |  |  |  |  |
APSC | Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease) amount | $ 2,700,000 | $ 0 | $ 0 |  |  |  | $ 0 |  | $ 2,500,000 |  | 2,700,000 | $ 4,800,000 |  |  | $ 10,700,000 | 7,800,000 | $ 4,700,000 |  |  |  |  |  |
Rate increase (decrease), margin of index range | 1.75% |  |  |  |  |  |  |  |  |  |  |  |  | 1.75% | 1.75% |  |  |  |  |  |  |  |
Bond purchase agreement amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 75,000,000 |  |  |  |
Approved return rate on equity |  |  |  |  |  |  |  |  |  |  |  |  | 10.80% |  |  |  |  |  |  |  |  |  |
Maximum return on equity, percentage of prior year revenues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.00%
Infrastructure System Replacement Surcharge | Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase, annualized revenue |  |  |  | $ 32,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory preferred stock and debt securities issuance, amount requested |  |  |  | 58,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Infrastructure System Replacement Surcharge | Missouri Gas Energy (MGE) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase, annualized revenue |  |  |  | 16,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory preferred stock and debt securities issuance, amount requested |  |  |  | $ 50,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Infrastructure System Replacement Surcharge | MoPSC | Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase |  |  |  |  |  | $ 3,300,000 |  |  |  |  | 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase, annualized revenue |  |  |  |  |  |  |  | $ 4,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease) amount |  |  |  |  | $ 32,600,000 |  |  | 29,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Infrastructure System Replacement Surcharge | MoPSC | Missouri Gas Energy (MGE) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase |  |  |  |  |  | $ 2,900,000 |  |  |  |  | 3,400,000 |  |  |  |  |  |  |  |  |  |  |  |
Recommended rate increase (decrease) amount |  |  |  |  | 3,000,000 |  |  |  |  | $ 3,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase, annualized revenue |  |  |  |  |  |  |  | 3,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease) amount |  |  |  |  | 16,400,000 |  |  | $ 13,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Updated filings | MoPSC | Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recommended rate increase (decrease) amount |  |  |  |  | $ 3,000,000 |  |  |  |  | $ 4,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Bond purchase agreement | Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Bond purchase agreement amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 170,000,000 |  |  |
Minimum | APSC | Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved return rate on equity |  |  |  |  |  |  |  |  |  |  |  |  | 10.50% |  |  |  |  |  |  |  |  |  |
Maximum | APSC | Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved return rate on equity |  |  |  |  |  |  |  |  |  |  |  |  | 10.95% |  |  |  |  |  |  |  |  |  |
Amount subject to refund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Current Regulatory Liabilities | $ 8,200,000 |  |  |  |  |  |  |  |  |  | 9,300,000 |  |  | $ 8,200,000 | $ 8,200,000 | 9,300,000 |  |  |  |  |  |  |
Amount subject to refund | Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Current Regulatory Liabilities | 0 |  |  |  |  |  |  |  |  |  | 0 |  |  | 0 | 0 | 0 |  |  |  |  |  |  |
Amount subject to refund | Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Current Regulatory Liabilities | 8,200,000 |  |  |  |  |  |  |  |  |  | $ 9,300,000 |  |  | 8,200,000 | 8,200,000 | $ 9,300,000 |  |  |  |  |  |  |
Amount subject to refund | APSC | Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Current Regulatory Liabilities | 6,300,000 |  |  |  |  |  |  |  |  |  |  |  |  | 6,300,000 | 6,300,000 |  |  |  |  |  |  |  |
Regulatory Liabilities | $ 12,300,000 |  |  |  |  |  |  |  |  |  |  |  |  | $ 12,300,000 | $ 12,300,000 |  |  |  |  |  |  |  |
Scenario, Forecast | Amount subject to refund | APSC | Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Current Regulatory Liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8,200,000 |  |  |  |  |

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