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Regulatory Matters - Spire Alabama - Additional Information (Details) - USD ($) |  |  |  | 1 Months Ended | 12 Months Ended |  |  |  |
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Dec. 01, 2019 | Oct. 01, 2018 | Jan. 01, 2014 | Nov. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2018 | Mar. 24, 2020 | Dec. 02, 2019 | Sep. 30, 2019 | Jan. 15, 2019
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Amounts returned to customers |  |  |  |  | $ 113,000,000.0 |  |  |  | $ 60,800,000 |
Spire Alabama |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Potential Performance-Based Adjustment, Percent |  |  |  |  | 0.05% |  |  |  |  |
Rate increase (decrease), margin of index range |  |  |  |  | 1.50% |  |  |  |  |
Amounts returned to customers |  |  |  |  | $ 3,900,000 |  |  |  | $ 3,400,000 |
Regulatory preferred stock and debt securities issuance, amount authorized |  |  |  |  |  |  | $ 150,000,000.0 |  |  |
APSC | ESR |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Extraordinary O&M expenses in excess of |  |  |  |  | 1,000,000.0 |  |  |  |  |
Single force majeure |  |  |  |  | 300,000 |  |  |  |  |
Multiple force majeures |  |  |  |  | 400,000 |  |  |  |  |
Revenue variances |  |  |  |  | $ 400,000 |  |  |  |  |
ESR term |  |  |  |  | 9 years |  |  |  |  |
Underfunded ESR balances amortization term |  |  |  |  | 5 years |  |  |  |  |
ESR annual limitation |  |  |  |  | $ 700,000 |  |  |  |  |
APSC | Spire Alabama |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved return rate on equity |  | 10.40% | 10.80% |  |  |  |  |  |  |
Maximum return on equity, percentage of prior year revenues |  |  |  |  | 4.00% |  |  |  |  |
Requested rate increase | $ 5,900,000 |  |  |  |  |  |  |  |  |
Return on equity threshold limit exceeded |  |  |  |  | 10.50% |  |  |  |  |
Approved percentage of excess natural gas supply and capacity retain |  |  |  | 25.00% |  |  |  |  |  |
Approved percentage of excess natural gas supply and capacity to customers |  |  |  | 75.00% |  |  |  |  |  |
Gas supply adjustment decrease on annual basis |  |  |  |  | $ 13,900,000 |  |  |  |  |
Rate increase (decrease), margin of index range |  |  |  |  |  |  |  |  | 1.75% |
Approved rate increase (decrease) amount |  |  |  |  | 5,900,000 | $ 9,700,000 |  |  |  |
Line of credit facility borrowing capacity |  |  |  |  | 200,000,000.0 |  |  |  |  |
Principal amount outstanding |  |  |  |  | 25,000,000.0 |  |  |  |  |
Regulatory preferred stock and debt securities issuance, amount authorized |  |  |  |  | $ 150,000,000.0 |  |  | $ 100,000,000.0 |  | $ 90,000,000.0
APSC | Spire Alabama | Amount subject to refund |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Amounts returned to customers |  |  |  |  |  | $ 7,200,000 |  |  | $ 4,200,000 |
AIM | Spire Alabama |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease) on allowed return on equity, percentage |  |  |  |  | 0.10% |  |  |  |  |
Minimum [Member] | APSC | Spire Alabama |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved return rate on equity |  | 10.15% | 10.50% |  |  |  |  |  |  |
Equity limitation as percent of capitalization |  |  | 55.50% |  |  |  |  |  |  |
Maximum | APSC | Spire Alabama |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |
Approved return rate on equity |  | 10.65% | 10.95% |  |  |  |  |  |  |
Equity limitation as percent of capitalization |  |  | 56.50% |  |  |  |  |  |  |

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