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Summary of Significant Accounting Policies - Additional Information (Details) | 6 Months Ended
--- | ---
Mar. 31, 2021 USD ($) reporting_unit | Mar. 31, 2021 USD ($) operating-segment
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |
Number of reportable segments | 2 | 2
Accounting Standards Update 2016-13 |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |
Credit loss allowances for available-for-sale securities recognized at adoption of ASU | $ 0 | $ 0
Change in accounting principle, accounting standards update, adopted | true | true
Change in accounting principle, accounting standards update, adoption date | Dec. 31, 2020 | Dec. 31, 2020
Change in accounting principle, accounting standards update, material effect | true | true
Accounting Standards Update 2016-13 | Spire Missouri |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |
Credit loss allowances for available-for-sale securities recognized at adoption of ASU | $ 0 | $ 0
Change in accounting principle, accounting standards update, adopted | true | true
Change in accounting principle, accounting standards update, adoption date | Dec. 31, 2020 | Dec. 31, 2020
Change in accounting principle, accounting standards update, material effect | true | true
Accounting Standards Update 2016-13 | Spire Alabama |  |
New Accounting Pronouncements Or Change In Accounting Principle [Line Items] |  |
Credit loss allowances for available-for-sale securities recognized at adoption of ASU | $ 0 | $ 0
Change in accounting principle, accounting standards update, adopted | true | true
Change in accounting principle, accounting standards update, adoption date | Dec. 31, 2020 | Dec. 31, 2020
Change in accounting principle, accounting standards update, material effect | true | true

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