Long-term Debt - Schedule of Long-Term Debt Instruments (Parentheticals) (Details)	12 Months Ended
Sep. 30, 2025
First Mortgage Bonds 3.40% Series, due March 15, 2028 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.40%
Debt instrument maturity date	Mar. 15, 2028
First Mortgage Bonds 7.00% Series, due June 1, 2029 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.00%
Debt instrument maturity date	Jun. 01, 2029
First Mortgage Bonds 2.84% Series, due November 15, 2029 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	2.84%
Debt instrument maturity date	Nov. 15, 2029
First Mortgage Bonds 4.88% Series, due September 15, 2030 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.88%
Debt instrument maturity date	Sep. 15, 2030
First Mortgage Bonds 7.90% Series, due September 15, 2030 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.90%
Debt instrument maturity date	Sep. 15, 2030
First Mortgage Bonds 5.12% Series, due September 15, 2032 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.12%
Debt instrument maturity date	Sep. 15, 2032
First Mortgage Bonds 3.68% Series, due September 15, 2032 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.68%
Debt instrument maturity date	Sep. 15, 2032
First Mortgage Bonds 4.80% Series, due February 15, 2033 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.80%
Debt instrument maturity date	Feb. 15, 2033
First Mortgage Bonds 6.00% Series, due May 1, 2034 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.00%
Debt instrument maturity date	May 01, 2034
First Mortgage Bonds 5.15% Series, due August 15, 2034 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.15%
Debt instrument maturity date	Aug. 15, 2034
First Mortgage Bonds 6.15% Series, due June 1, 2036 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.15%
Debt instrument maturity date	Jun. 01, 2036
First Mortgage Bonds 4.63% Series, due August 15, 2043 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.63%
Debt instrument maturity date	Aug. 15, 2043
First Mortgage Bonds 4.23% Series, due September 15, 2047 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.23%
Debt instrument maturity date	Sep. 15, 2047
First Mortgage Bonds 3.30% Series, due June 1, 2051 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.30%
Debt instrument maturity date	Jun. 01, 2051
First Mortgage Bonds 4.38% Series, due September 15, 2057 [Member] \| Spire Missouri [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.38%
Debt instrument maturity date	Sep. 15, 2057
3.21% Notes, due September 15, 2025 [Member] \| Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.21%
Debt instrument maturity date	Sep. 15, 2025
5.32% Notes, due October 15, 2029 [Member] \| Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.32%
Debt instrument maturity date	Oct. 15, 2029
2.88% Notes, due December 1, 2029 [Member] \| Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	2.88%
Debt instrument maturity date	Dec. 01, 2029
2.04% Notes, due December 15, 2030 [Member] \| Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	2.04%
Debt instrument maturity date	Dec. 15, 2030
5.41% Notes, due October 15, 2032 [Member] \| Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.41%
Debt instrument maturity date	Oct. 15, 2032
5.90% Notes, due January 15, 2037 [Member] \| Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.90%
Debt instrument maturity date	Jan. 15, 2037
4.31% Notes, due December 1, 2045 [Member] \| Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.31%
Debt instrument maturity date	Dec. 01, 2045
3.92% Notes, due January 15, 2048 [Member] \| Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.92%
Debt instrument maturity date	Jan. 15, 2048
4.64% Notes, due January 15, 2049 [Member] \| Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.64%
Debt instrument maturity date	Jan. 15, 2049
4.02% Notes, due January 15, 2058 [Member] \| Spire Alabama Inc [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.02%
Debt instrument maturity date	Jan. 15, 2058
Parent Company [Member] \| 5.30% Senior Notes, due March 1, 2026 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.30%
Debt instrument maturity date	Mar. 01, 2026
Parent Company [Member] \| 3.13% Senior Notes, due September 1, 2026 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.13%
Debt instrument maturity date	Sep. 01, 2026
Parent Company [Member] \| 3.93% Senior Notes, due March 15, 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.93%
Debt instrument maturity date	Mar. 15, 2027
Parent Company [Member] \| 5.80% Senior Notes, due March 15, 2033 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.80%
Debt instrument maturity date	Mar. 15, 2033
Parent Company [Member] \| 4.70% Senior Notes, due August 15, 2044 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.70%
Debt instrument maturity date	Aug. 15, 2044
Subsidiaries [Member] \| 5.00% First Mortgage Bonds, due September 30, 2031 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.00%
Debt instrument maturity date	Sep. 30, 2031
Subsidiaries [Member] \| 2.95% Notes, with annual principal payments through December 2034 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	2.95%
Debt instrument maturity date	Dec. 31, 2034
Subsidiaries [Member] \| 5.61% First Mortgage Bonds, due October 15, 2037 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.61%
Debt instrument maturity date	Oct. 15, 2037
Subsidiaries [Member] \| 3.52% First Mortgage Bonds, due September 30, 2049 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.52%
Debt instrument maturity date	Sep. 30, 2049

First Mortgage Bonds 3.40% Series, due March 15, 2028 [Member] | Spire Missouri [Member]