Long-term Debt - Schedule of Composition of Long-term Debt (Parenthetical) (Details)	12 Months Ended
	Sep. 30, 2020
Spire Missouri | Floating-rate note due December 1, 2021
Debt Instrument [Line Items]
Debt instrument, maturity date	Dec. 01, 2021
Spire Missouri | 3.00% Series, due March 15, 2023
Debt Instrument [Line Items]
Debt instrument, interest rate	3.00%
Debt instrument, maturity date	Mar. 15, 2023
Spire Missouri | 3.40% Series, due August 15, 2023
Debt Instrument [Line Items]
Debt instrument, interest rate	3.40%
Debt instrument, maturity date	Aug. 15, 2023
Spire Missouri | 3.40% Series, due March 15, 2028
Debt Instrument [Line Items]
Debt instrument, interest rate	3.40%
Debt instrument, maturity date	Mar. 15, 2028
Spire Missouri | 2.84% Series, due November 15, 2029
Debt Instrument [Line Items]
Debt instrument, interest rate	2.84%
Debt instrument, maturity date	Nov. 15, 2029
Spire Missouri | 7.0% Series, due June 1, 2029
Debt Instrument [Line Items]
Debt instrument, interest rate	7.00%
Debt instrument, maturity date	Jun. 01, 2029
Spire Missouri | 7.90% Series, due September 15, 2030
Debt Instrument [Line Items]
Debt instrument, interest rate	7.90%
Debt instrument, maturity date	Sep. 15, 2030
Spire Missouri | 3.68% Series, due September 15, 2032
Debt Instrument [Line Items]
Debt instrument, interest rate	3.68%
Debt instrument, maturity date	Sep. 15, 2032
Spire Missouri | 6.00% Series, due May 1, 2034
Debt Instrument [Line Items]
Debt instrument, interest rate	6.00%
Debt instrument, maturity date	May 01, 2034
Spire Missouri | 6.15% Series, due June 1, 2036
Debt Instrument [Line Items]
Debt instrument, interest rate	6.15%
Debt instrument, maturity date	Jun. 01, 2036
Spire Missouri | 4.63% Series, due August 15, 2043
Debt Instrument [Line Items]
Debt instrument, interest rate	4.63%
Debt instrument, maturity date	Aug. 15, 2043
Spire Missouri | 4.23% Series, due September 15, 2047
Debt Instrument [Line Items]
Debt instrument, interest rate	4.23%
Debt instrument, maturity date	Sep. 15, 2047
Spire Missouri | 4.38% Series, due September 15, 2057
Debt Instrument [Line Items]
Debt instrument, interest rate	4.38%
Debt instrument, maturity date	Sep. 15, 2057
Spire Alabama | 5.20% Notes, due January 15, 2020
Debt Instrument [Line Items]
Debt instrument, interest rate	5.20%
Debt instrument, maturity date	Jan. 15, 2020
Spire Alabama | 3.86% Notes, due December 22, 2021
Debt Instrument [Line Items]
Debt instrument, interest rate	3.86%
Debt instrument, maturity date	Dec. 22, 2021
Spire Alabama | 3.21% Notes, due September 15, 2025
Debt Instrument [Line Items]
Debt instrument, interest rate	3.21%
Debt instrument, maturity date	Sep. 15, 2025
Spire Alabama | 2.88% Notes, due December 1, 2029
Debt Instrument [Line Items]
Debt instrument, interest rate	2.88%
Debt instrument, maturity date	Dec. 01, 2029
Spire Alabama | 5.90% Notes, due January 15, 2037
Debt Instrument [Line Items]
Debt instrument, interest rate	5.90%
Debt instrument, maturity date	Jan. 15, 2037
Spire Alabama | 4.31% Notes, due December 1, 2045
Debt Instrument [Line Items]
Debt instrument, interest rate	4.31%
Debt instrument, maturity date	Dec. 01, 2045
Spire Alabama | 3.92% Notes, due January 15, 2048
Debt Instrument [Line Items]
Debt instrument, interest rate	3.92%
Debt instrument, maturity date	Jan. 15, 2048
Spire Alabama | 4.64% Notes, due January 15, 2049
Debt Instrument [Line Items]
Debt instrument, interest rate	4.64%
Debt instrument, maturity date	Jan. 15, 2049
Spire Alabama | 4.02% Notes, due January 15, 2058
Debt Instrument [Line Items]
Debt instrument, interest rate	4.02%
Debt instrument, maturity date	Jan. 15, 2058
Spire | 2.52% Senior Notes, due September 1, 2021
Debt Instrument [Line Items]
Debt instrument, interest rate	2.52%
Debt instrument, maturity date	Sep. 01, 2021
Spire | 3.31% Notes Payable, due December 15, 2022
Debt Instrument [Line Items]
Debt instrument, interest rate	3.31%
Debt instrument, maturity date	Dec. 15, 2022
Spire | 3.54% Senior Notes, due February 27, 2024
Debt Instrument [Line Items]
Debt instrument, interest rate	3.54%
Debt instrument, maturity date	Feb. 27, 2024
Spire | 3.13% Senior Notes, due September 1, 2026
Debt Instrument [Line Items]
Debt instrument, interest rate	3.13%
Debt instrument, maturity date	Sep. 01, 2026
Spire | 3.93% Senior Notes, due March 15, 2027
Debt Instrument [Line Items]
Debt instrument, interest rate	3.93%
Debt instrument, maturity date	Mar. 15, 2027
Spire | 4.70% Senior Notes, due August 15, 2044
Debt Instrument [Line Items]
Debt instrument, interest rate	4.70%
Debt instrument, maturity date	Aug. 15, 2044
Subsidiaries | 4.14% First Mortgage Bonds, due September 30, 2021
Debt Instrument [Line Items]
Debt instrument, interest rate	4.14%
Debt instrument, maturity date	Sep. 30, 2021
Subsidiaries | 5.00% First Mortgage Bonds, due September 30, 2031
Debt Instrument [Line Items]
Debt instrument, interest rate	5.00%
Debt instrument, maturity date	Sep. 30, 2031
Subsidiaries | 2.95% Notes, with Annual Principal Payments through December 2034
Debt Instrument [Line Items]
Debt instrument, interest rate	2.95%
Debt instrument, maturity date	Dec. 31, 2034
Subsidiaries | 3.52% First Mortgage Bonds, due September 30, 2049
Debt Instrument [Line Items]
Debt instrument, interest rate	3.52%
Debt instrument, maturity date	Sep. 30, 2049