Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||||
|---|---|---|---|---|---|---|
Mar. 31, 2026 |
Mar. 31, 2026 |
Mar. 31, 2025 |
Mar. 31, 2026 |
Mar. 31, 2025 |
Sep. 30, 2025 |
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| Accounting Policies [Line Items] | ||||||
| Goodwill | $ 1,891.4 | $ 1,891.4 | $ 1,171.6 | $ 1,891.4 | $ 1,171.6 | $ 1,171.6 |
| Goodwill impairment | 3.9 | 0.0 | 3.9 | 0.0 | ||
| Restricted cash | 36.1 | 36.1 | 30.8 | 36.1 | 30.8 | 35.5 |
| Spire Missouri [Member] | ||||||
| Accounting Policies [Line Items] | ||||||
| Goodwill | 210.2 | 210.2 | $ 210.2 | 210.2 | $ 210.2 | 210.2 |
| Southeast Utilities [Member] | ||||||
| Accounting Policies [Line Items] | ||||||
| Goodwill | 18.1 | 18.1 | 18.1 | $ 961.4 | ||
| Spire Tennessee [Member] | ||||||
| Accounting Policies [Line Items] | ||||||
| Goodwill | 737.9 | 737.9 | 737.9 | |||
| Spire Mississippi [Member] | ||||||
| Accounting Policies [Line Items] | ||||||
| Goodwill impairment | 3.9 | |||||
| Piedmont Tennessee Transaction [Member] | ||||||
| Accounting Policies [Line Items] | ||||||
| Goodwill | 737.9 | $ 737.9 | 737.9 | |||
| Payments to acquire businesses, gross | $ 2,500.0 | $ 2,500.0 | ||||
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- Definition Accounting Policies No definition available.
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- Definition Amount, after accumulated impairment loss, of asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized. Reference 1: http://www.xbrl.org/2003/role/exampleRef
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- Definition Amount of impairment loss from asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. Reference 1: http://www.xbrl.org/2003/role/exampleRef
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- Definition Amount of cash restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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