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v3.23.3

Note 6 - Long-term Debt - Composition of Long-term Debt (Details) (Parentheticals) | 12 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022
First Mortgage Bonds Three Point Four Percentage Series Due August152023 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 3.40% | 3.40%
Debt Instrument Maturity Date | Aug. 15, 2023 | Aug. 15, 2023
Floating Rate Series Due December 2, 2024 [Member] | Spire Missouri [Member] |  |
Debt Instrument Maturity Date | Dec. 02, 2024 |
First Mortgage Bonds Three Point Four Percentage Series Due March152028 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 3.40% | 3.40%
Debt Instrument Maturity Date | Mar. 15, 2028 | Mar. 15, 2028
First Mortgage Bonds Seven Percentage Series Due June12029 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 7.00% | 7.00%
Debt Instrument Maturity Date | Jun. 01, 2029 | Jun. 01, 2029
First Mortgage Bonds Two Point Eight Four Percentage Series Due November152029 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 2.84% | 2.84%
Debt Instrument Maturity Date | Nov. 15, 2029 | Nov. 15, 2029
First Mortgage Bonds Seven Point Nine Percentage Series Due September152030 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 7.90% | 7.90%
Debt Instrument Maturity Date | Sep. 15, 2030 | Sep. 15, 2030
First Mortgage Bonds Three Point Six Eight Percentage Series Due September152032 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 3.68% | 3.68%
Debt Instrument Maturity Date | Sep. 15, 2032 | Sep. 15, 2032
First Mortgage Bonds Four Point Eight Zero Percentage Series Due February 152033 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 4.80% | 4.80%
Debt Instrument Maturity Date | Feb. 15, 2033 | Feb. 15, 2033
First Mortgage Bonds Six Percentage Series Due May12034 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 6.00% | 6.00%
Debt Instrument Maturity Date | May 01, 2034 | May 01, 2034
First Mortgage Bonds Six Point One Five Percentage Series Due June12036 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 6.15% | 6.15%
Debt Instrument Maturity Date | Jun. 01, 2036 | Jun. 01, 2036
First Mortgage Bonds Four Point Six Three Percentage Series Due August152043 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 4.63% | 4.63%
Debt Instrument Maturity Date | Aug. 15, 2043 | Aug. 15, 2043
First Mortgage Bonds Four Point Two Three Percentage Series Due September152047 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 4.23% | 4.23%
Debt Instrument Maturity Date | Sep. 15, 2047 | Sep. 15, 2047
First Mortgage Bonds Three Point Three Zero Percentage Series Due June12051 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 3.30% | 3.30%
Debt Instrument Maturity Date | Jun. 01, 2051 | Jun. 01, 2051
First Mortgage Bonds Four Point Three Eight Percentage Series Due September152057 [Member] | Spire Missouri [Member] |  |
Debt Instrument Interest Rate | 4.38% | 4.38%
Debt Instrument Maturity Date | Sep. 15, 2057 | Sep. 15, 2057
Three Point Two One Percentage Notes Due September152025 [Member] | Spire Alabama Inc [Member] |  |
Debt Instrument Interest Rate | 3.21% | 3.21%
Debt Instrument Maturity Date | Sep. 15, 2025 | Sep. 15, 2025
Five Point Three Two Percentage Notes Due October152029 [Member] | Spire Alabama Inc [Member] |  |
Debt Instrument Interest Rate | 5.32% | 5.32%
Debt Instrument Maturity Date | Oct. 15, 2029 | Oct. 15, 2029
Two Point Eight Eight Percentage Notes Due December12029 [Member] | Spire Alabama Inc [Member] |  |
Debt Instrument Interest Rate | 2.88% | 2.88%
Debt Instrument Maturity Date | Dec. 01, 2029 | Dec. 01, 2029
Two Point Zero Four Percentage Notes Due December152030 [Member] | Spire Alabama Inc [Member] |  |
Debt Instrument Interest Rate | 2.04% | 2.04%
Debt Instrument Maturity Date | Dec. 15, 2030 | Dec. 15, 2030
Five Point Four One Percentage Notes Due October152032[Member] | Spire Alabama Inc [Member] |  |
Debt Instrument Interest Rate | 5.41% | 5.41%
Debt Instrument Maturity Date | Oct. 15, 2032 | Oct. 15, 2032
Five Point Nine Zero Percentage Notes Due January152037 [Member] | Spire Alabama Inc [Member] |  |
Debt Instrument Interest Rate | 5.90% | 5.90%
Debt Instrument Maturity Date | Jan. 15, 2037 | Jan. 15, 2037
Four Point Three One Percentage Notes Due December12045 [Member] | Spire Alabama Inc [Member] |  |
Debt Instrument Interest Rate | 4.31% | 4.31%
Debt Instrument Maturity Date | Dec. 01, 2045 | Dec. 01, 2045
Three Point Nine Two Percentage Notes Due January152048 [Member] | Spire Alabama Inc [Member] |  |
Debt Instrument Interest Rate | 3.92% | 3.92%
Debt Instrument Maturity Date | Jan. 15, 2048 | Jan. 15, 2048
Four Point Six Four Percentage Notes Due January152049 [Member] | Spire Alabama Inc [Member] |  |
Debt Instrument Interest Rate | 4.64% | 4.64%
Debt Instrument Maturity Date | Jan. 15, 2049 | Jan. 15, 2049
Four Point Zero Two Percentage Notes Due January152058 [Member] | Spire Alabama Inc [Member] |  |
Debt Instrument Interest Rate | 4.02% | 4.02%
Debt Instrument Maturity Date | Jan. 15, 2058 | Jan. 15, 2058
Parent Company [Member] | Three Point Three One Percent Notes Payable Due December152022 [Member] |  |
Debt Instrument Interest Rate | 3.31% | 3.31%
Debt Instrument Maturity Date | Dec. 15, 2022 | Dec. 15, 2022
Parent Company [Member] | Three Point Five Four Percent Senior Notes Due February272024 [Member] |  |
Debt Instrument Interest Rate | 3.54% | 3.54%
Debt Instrument Maturity Date | Feb. 27, 2024 | Feb. 27, 2024
Parent Company [Member] | Zero Point Seven Five Percent Remarketable Senior Notes Due March12026 [Member] |  |
Debt Instrument Interest Rate | 0.75% | 0.75%
Debt Instrument Maturity Date | Mar. 01, 2026 | Mar. 01, 2026
Parent Company [Member] | Three Point One Three Percent Senior Notes Due September12026 [Member] |  |
Debt Instrument Interest Rate | 3.13% | 3.13%
Debt Instrument Maturity Date | Sep. 01, 2026 | Sep. 01, 2026
Parent Company [Member] | Three Point Nine Three Percent Senior Notes Due March152027 [Member] |  |
Debt Instrument Interest Rate | 3.93% | 3.93%
Debt Instrument Maturity Date | Mar. 15, 2027 | Mar. 15, 2027
Parent Company [Member] | Five Point Eight Percent Senior Notes Due March152033 [Member] |  |
Debt Instrument Interest Rate | 5.80% | 5.80%
Debt Instrument Maturity Date | Mar. 15, 2033 | Mar. 15, 2033
Parent Company [Member] | Four Point Seven Zero Percent Senior Notes Due August152044 [Member] |  |
Debt Instrument Interest Rate | 4.70% | 4.70%
Debt Instrument Maturity Date | Aug. 15, 2044 | Aug. 15, 2044
Subsidiaries [Member] | Five Point Zero Zero Percent First Mortgage Bonds Due September302031 [Member] |  |
Debt Instrument Interest Rate | 5.00% | 5.00%
Debt Instrument Maturity Date | Sep. 30, 2031 | Sep. 30, 2031
Subsidiaries [Member] | Two Nine Five Percent Notes With Annual Principal Payments Through December2034 [Member] |  |
Debt Instrument Interest Rate | 2.95% | 2.95%
Debt Instrument Maturity Date | Dec. 31, 2034 | Dec. 31, 2034
Subsidiaries [Member] | Five Point Six One Percent First Mortgage Bonds Due October 152037 [Member] |  |
Debt Instrument Interest Rate | 5.61% | 5.61%
Debt Instrument Maturity Date | Oct. 15, 2037 | Oct. 15, 2037
Subsidiaries [Member] | Three Point Five Two Percent First Mortgage Bonds Due September302049 [Member] |  |
Debt Instrument Interest Rate | 3.52% | 3.52%
Debt Instrument Maturity Date | Sep. 30, 2049 | Sep. 30, 2049

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