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Income Taxes - Effective Income Tax Rate Variation from Stated Tax Rate (Details) | 3 Months Ended | 12 Months Ended
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Dec. 31, 2017 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Income Tax Contingency [Line Items] |  |  |  |
Federal income tax statutory rate | 35.00% | 21.00% |  |
Spire |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 24.50%
State and local income taxes, net of federal income tax benefits |  | 9.00% | 3.60% | 3.40%
Certain expenses capitalized on books and deducted on tax return |  | (6.60%) | (3.80%) | (2.30%)
Taxes related to prior years |  | (1.80%) | 0.20% | (0.40%)
Tax law changes |  | 0.00% | 0.00% | (35.90%)
Amortization of excess deferred taxes |  | (8.30%) | (3.80%) | (1.80%)
Other items – net |  | (1.00%) | (1.40%) | (1.60%)
Effective income tax rate |  | 12.30% | 15.80% | (14.10%)
Spire Missouri |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 24.50%
State and local income taxes, net of federal income tax benefits |  | 2.60% | 2.60% | 3.40%
Certain expenses capitalized on books and deducted on tax return |  | (4.60%) | (6.50%) | (4.60%)
Taxes related to prior years |  | (1.40%) | 0.10% | (0.70%)
Tax law changes |  | 0.00% | 0.00% | (50.30%)
Amortization of excess deferred taxes |  | (5.70%) | (6.60%) | (3.60%)
Other items – net |  | (0.20%) | (0.30%) | (2.50%)
Effective income tax rate |  | 11.70% | 10.30% | (33.80%)
Spire Alabama |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |
Federal income tax statutory rate |  | 21.00% | 21.00% | 24.50%
State and local income taxes, net of federal income tax benefits |  | 4.10% | 4.10% | 3.80%
Certain expenses capitalized on books and deducted on tax return |  | 0.00% | 0.00% | 0.00%
Taxes related to prior years |  | 0.10% | 0.00% | 0.00%
Tax law changes |  | 0.00% | 0.00% | 70.00%
Amortization of excess deferred taxes |  | 0.00% | 0.00% | 0.00%
Other items – net |  | (0.10%) | 0.20% | 0.10%
Effective income tax rate |  | 25.10% | 25.30% | 98.40%

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- Definition Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319 + Details
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