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Long-term Debt (Tables) | 12 Months Ended
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Sep. 30, 2025
Debt Disclosure [Abstract] |
Schedule of Long-Term Debt Instruments | The composition of long-term debt as of September 30 is shown in the following tables.
|  | 2025 |  |  | 2024 |
Spire |  |  |  |  |  |
5.30% Senior Notes, due March 1, 2026 |  | $ | 350.0 |  |  | $ | 350.0 |
3.13% Senior Notes, due September 1, 2026 |  |  | 130.0 |  |  |  | 130.0 |
3.93% Senior Notes, due March 15, 2027 |  |  | 100.0 |  |  |  | 100.0 |
5.80% Senior Notes, due March 15, 2033 |  |  | 150.0 |  |  |  | 150.0 |
4.70% Senior Notes, due August 15, 2044 |  |  | 250.0 |  |  |  | 250.0 |
Total principal of Spire Missouri long-term debt (see below) |  |  | 1,968.0 |  |  |  | 1,818.0 |
Total principal of Spire Alabama long-term debt (see below) |  |  | 715.0 |  |  |  | 750.0 |
Other subsidiaries' long-term debt: |  |  |  |  |  |
5.00% First Mortgage Bonds, due September 30, 2031 |  |  | 42.0 |  |  |  | 42.0 |
2.95% Notes, with annual principal payments through December 2034 |  |  | 104.1 |  |  |  | 111.1 |
5.61% First Mortgage Bonds, due October 15, 2037 |  |  | 30.0 |  |  |  | 30.0 |
3.52% First Mortgage Bonds, due September 30, 2049 |  |  | 40.0 |  |  |  | 40.0 |
Total principal of long-term debt |  |  | 3,879.1 |  |  |  | 3,771.1 |
Less: Unamortized discounts and debt issuance costs |  |  | (22.2 | ) |  |  | (24.7 | )
Less: Current portion |  |  | (487.5 | ) |  |  | (42.0 | )
Long-term debt, excluding current portion |  | $ | 3,369.4 |  |  | $ | 3,704.4 |
Spire Missouri |  |  |  |  |  |
First Mortgage Bonds: |  |  |  |  |  |
3.40% Series, due March 15, 2028 |  | $ | 45.0 |  |  | $ | 45.0 |
7.00% Series, due June 1, 2029 |  |  | 19.3 |  |  |  | 19.3 |
2.84% Series, due November 15, 2029 |  |  | 275.0 |  |  |  | 275.0 |
4.88% Series, due September 15, 2030 |  |  | 90.0 |  |  |  | — |
7.90% Series, due September 15, 2030 |  |  | 30.0 |  |  |  | 30.0 |
5.12% Series, due September 15, 2032 |  |  | 60.0 |  |  |  | — |
3.68% Series, due September 15, 2032 |  |  | 50.0 |  |  |  | 50.0 |
4.80% Series, due February 15, 2033 |  |  | 400.0 |  |  |  | 400.0 |
6.00% Series, due May 1, 2034 |  |  | 99.3 |  |  |  | 99.3 |
5.15% Series, due August 15, 2034 |  |  | 320.0 |  |  |  | 320.0 |
6.15% Series, due June 1, 2036 |  |  | 54.5 |  |  |  | 54.5 |
4.63% Series, due August 15, 2043 |  |  | 99.9 |  |  |  | 99.9 |
4.23% Series, due September 15, 2047 |  |  | 70.0 |  |  |  | 70.0 |
3.30% Series, due June 1, 2051 |  |  | 305.0 |  |  |  | 305.0 |
4.38% Series, due September 15, 2057 |  |  | 50.0 |  |  |  | 50.0 |
Total principal of Spire Missouri long-term debt |  |  | 1,968.0 |  |  |  | 1,818.0 |
Less: Unamortized discounts and debt issuance costs |  |  | (14.4 | ) |  |  | (14.6 | )
Spire Missouri long-term debt, excluding current portion |  | $ | 1,953.6 |  |  | $ | 1,803.4 |
Spire Alabama |  |  |  |  |  |
3.21% Notes, due September 15, 2025 |  | $ | — |  |  | $ | 35.0 |
5.32% Notes, due October 15, 2029 |  |  | 90.0 |  |  |  | 90.0 |
2.88% Notes, due December 1, 2029 |  |  | 100.0 |  |  |  | 100.0 |
2.04% Notes, due December 15, 2030 |  |  | 150.0 |  |  |  | 150.0 |
5.41% Notes, due October 15, 2032 |  |  | 85.0 |  |  |  | 85.0 |
5.90% Notes, due January 15, 2037 |  |  | 45.0 |  |  |  | 45.0 |
4.31% Notes, due December 1, 2045 |  |  | 80.0 |  |  |  | 80.0 |
3.92% Notes, due January 15, 2048 |  |  | 45.0 |  |  |  | 45.0 |
4.64% Notes, due January 15, 2049 |  |  | 90.0 |  |  |  | 90.0 |
4.02% Notes, due January 15, 2058 |  |  | 30.0 |  |  |  | 30.0 |
Total principal of Spire Alabama long-term debt |  |  | 715.0 |  |  |  | 750.0 |
Less: Unamortized discounts and debt issuance costs |  |  | (3.3 | ) |  |  | (3.7 | )
Less: Current portion |  |  | — |  |  |  | (35.0 | )
Spire Alabama long-term debt, excluding current portion |  | $ | 711.7 |  |  | $ | 711.3 |
Schedule of Maturities of Long-Term Debt | Maturities of long-term debt for Spire on a consolidated basis, Spire Missouri and Spire Alabama for the five fiscal years after September 30, 2025 are as follows:
|  | 2026 |  |  | 2027 |  |  | 2028 |  |  | 2029 |  |  | 2030 |
Spire |  | $ | 487.5 |  |  | $ | 108.1 |  |  | $ | 53.6 |  |  | $ | 28.5 |  |  | $ | 594.9 |
Spire Missouri |  |  | — |  |  |  | — |  |  |  | 45.0 |  |  |  | 19.3 |  |  |  | 395.0 |
Spire Alabama |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 190.0 |

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