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