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Long-term Debt - Schedule of Long-Term Debt Instruments (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
--- | --- | ---
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 (less current portion) | 3,369.4 | 3,704.4
Spire Missouri [Member] |  |
Total principal of long-term debt | 1,968.0 | 1,818.0
Less: Unamortized discounts and debt issuance costs | (14.4) | (14.6)
Long-term debt (less current portion) | 1,953.6 | 1,803.4
Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 715.0 | 750.0
Less: Unamortized discounts and debt issuance costs | (3.3) | (3.7)
Less: Current portion | 0.0 | (35.0)
Long-term debt (less current portion) | 711.7 | 711.3
First Mortgage Bonds 3.40% Series, due March 15, 2028 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 45.0 | 45.0
First Mortgage Bonds 7.00% Series, due June 1, 2029 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 19.3 | 19.3
First Mortgage Bonds 2.84% Series, due November 15, 2029 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 275.0 | 275.0
First Mortgage Bonds 4.88% Series, due September 15, 2030 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 90.0 | 0.0
First Mortgage Bonds 7.90% Series, due September 15, 2030 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 30.0 | 30.0
First Mortgage Bonds 5.12% Series, due September 15, 2032 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 60.0 | 0.0
First Mortgage Bonds 3.68% Series, due September 15, 2032 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 50.0 | 50.0
First Mortgage Bonds 4.80% Series, due February 15, 2033 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 400.0 | 400.0
First Mortgage Bonds 6.00% Series, due May 1, 2034 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 99.3 | 99.3
First Mortgage Bonds 5.15% Series, due August 15, 2034 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 320.0 | 320.0
First Mortgage Bonds 6.15% Series, due June 1, 2036 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 54.5 | 54.5
First Mortgage Bonds 4.63% Series, due August 15, 2043 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 99.9 | 99.9
First Mortgage Bonds 4.23% Series, due September 15, 2047 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 70.0 | 70.0
First Mortgage Bonds 3.30% Series, due June 1, 2051 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 305.0 | 305.0
First Mortgage Bonds 4.38% Series, due September 15, 2057 [Member] | Spire Missouri [Member] |  |
Total principal of long-term debt | 50.0 | 50.0
3.21% Notes, due September 15, 2025 [Member] | Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 0.0 | 35.0
5.32% Notes, due October 15, 2029 [Member] | Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 90.0 | 90.0
2.88% Notes, due December 1, 2029 [Member] | Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 100.0 | 100.0
2.04% Notes, due December 15, 2030 [Member] | Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 150.0 | 150.0
5.41% Notes, due October 15, 2032 [Member] | Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 85.0 | 85.0
5.90% Notes, due January 15, 2037 [Member] | Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 45.0 | 45.0
4.31% Notes, due December 1, 2045 [Member] | Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 80.0 | 80.0
3.92% Notes, due January 15, 2048 [Member] | Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 45.0 | 45.0
4.64% Notes, due January 15, 2049 [Member] | Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 90.0 | 90.0
4.02% Notes, due January 15, 2058 [Member] | Spire Alabama Inc [Member] |  |
Total principal of long-term debt | 30.0 | 30.0
Parent Company [Member] | 5.30% Senior Notes, due March 1, 2026 [Member] |  |
Total principal of long-term debt | 350.0 | 350.0
Parent Company [Member] | 3.13% Senior Notes, due September 1, 2026 [Member] |  |
Total principal of long-term debt | 130.0 | 130.0
Parent Company [Member] | 3.93% Senior Notes, due March 15, 2027 [Member] |  |
Total principal of long-term debt | 100.0 | 100.0
Parent Company [Member] | 5.80% Senior Notes, due March 15, 2033 [Member] |  |
Total principal of long-term debt | 150.0 | 150.0
Parent Company [Member] | 4.70% Senior Notes, due August 15, 2044 [Member] |  |
Total principal of long-term debt | 250.0 | 250.0
Subsidiaries [Member] | 5.00% First Mortgage Bonds, due September 30, 2031 [Member] |  |
Total principal of long-term debt | 42.0 | 42.0
Subsidiaries [Member] | 2.95% Notes, with annual principal payments through December 2034 [Member] |  |
Total principal of long-term debt | 104.1 | 111.1
Subsidiaries [Member] | 5.61% First Mortgage Bonds, due October 15, 2037 [Member] |  |
Total principal of long-term debt | 30.0 | 30.0
Subsidiaries [Member] | 3.52% First Mortgage Bonds, due September 30, 2049 [Member] |  |
Total principal of long-term debt | $ 40.0 | $ 40.0

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