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Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
--- | --- | ---
Total | $ 52.1 | $ 43.3
Total | 38.6 | 32.3
Fair Value, Recurring [Member] |  |
Effects of netting, asset | (17.0) | (13.9)
Total | $ 131.6 | $ 113.3
Derivative Asset, Statement of Financial Position [Extensible Enumeration] | Other Investments | Other Investments
Effects of netting, liability | $ (23.0) | $ (25.0)
Total | $ 38.6 | $ 32.3
Derivative Liability, Statement of Financial Position [Extensible Enumeration] | Other libilities | Other libilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Derivative asset fair value gross asset | $ 93.8 | $ 80.4
Derivative liability fair value gross liability | 44.9 | 31.3
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Derivative asset fair value gross asset | 54.8 | 46.8
Derivative liability fair value gross liability | 16.7 | 26.0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Derivative liability fair value gross liability | 0.0 | 0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Gas Utility [Member] |  |
Effects of netting, asset | 0.0 | 0.0
Total | 27.5 | 24.3
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Other Operating Segment [Member] |  |
Effects of netting, asset | 0.0 | 0.0
Total | 27.0 | 17.9
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member] |  |
Derivative asset fair value gross asset | 27.5 | 24.3
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 27.0 | 17.9
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Utility [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Gas Utility [Member] |  |
Effects of netting, asset | (2.9) | (3.4)
Total | 0.0 | 0.0
Effects of netting, liability | (1.3) | (3.8)
Total | 24.6 | 9.8
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Gas Marketing [Member] |  |
Effects of netting, asset | (11.4) | (7.0)
Total | 0.0 | 0.0
Effects of netting, liability | (19.0) | (17.7)
Total | 0.0 | 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member] |  |
Derivative asset fair value gross asset | 2.9 | 3.4
Derivative liability fair value gross liability | 25.9 | 13.6
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member] |  |
Derivative asset fair value gross asset | 11.4 | 7.0
Derivative liability fair value gross liability | 19.0 | 17.7
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Derivative liability fair value gross liability | 0.0 | 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Derivative liability fair value gross liability | 0.0 | 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Utility [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Derivative liability fair value gross liability | 0.0 | 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Marketing [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Derivative liability fair value gross liability | 0.0 | 0.0
Natural Gas Commodity Contracts [Member] | Fair Value, Recurring [Member] | Gas Marketing [Member] |  |
Effects of netting, asset | (2.7) | (3.5)
Total | 42.1 | 42.5
Effects of netting, liability | (2.7) | (3.5)
Total | 14.0 | 21.0
Natural Gas Commodity Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Derivative liability fair value gross liability | 0.0 | 0.0
Natural Gas Commodity Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member] |  |
Derivative asset fair value gross asset | 44.8 | 46.0
Derivative liability fair value gross liability | 16.7 | 24.5
Natural Gas Commodity Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Marketing [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Derivative liability fair value gross liability | 0.0 | 0.0
U.S. Bonds [Member] | Fair Value, Recurring [Member] | Other Operating Segment [Member] |  |
Total | 23.5 | 21.9
U.S. Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 23.5 | 21.9
U.S. Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
U.S. Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Global Bonds [Member] | Fair Value, Recurring [Member] | Other Operating Segment [Member] |  |
Effects of netting, asset | 0.0 | 0.0
Total | 1.5 | 5.9
Global Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 1.5 | 5.9
Global Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Global Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Other Operating Segment [Member] |  |
Effects of netting, asset | 0.0 | 0.0
Total | 10.0 | 0.8
Effects of netting, liability |  | 0.0
Total |  | 1.5
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Derivative liability fair value gross liability |  | 0.0
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 10.0 | 0.8
Derivative liability fair value gross liability |  | 1.5
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Operating Segment [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Derivative liability fair value gross liability |  | 0.0
Spire Missouri [Member] |  |
Total | 0.0 | 0.0
Total | 24.6 | 9.8
Spire Missouri [Member] | Fair Value, Recurring [Member] |  |
Effects of netting, asset | (2.9) | (3.4)
Total | $ 27.5 | $ 24.3
Derivative Asset, Statement of Financial Position [Extensible Enumeration] | Other Property and Investments | Other Property and Investments
Spire Missouri [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Derivative asset fair value gross asset | $ 30.4 | $ 27.7
Spire Missouri [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Spire Missouri [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] |  |
Effects of netting, asset | 0.0 | 0.0
Total | 27.5 | 24.3
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Derivative asset fair value gross asset | 27.5 | 24.3
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] |  |
Effects of netting, asset | (2.9) | (3.4)
Total | 0.0 | 0.0
Effects of netting, liability | (1.3) | (3.8)
Total | 24.6 | 9.8
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Derivative asset fair value gross asset | 2.9 | 3.4
Derivative liability fair value gross liability | 25.9 | 13.6
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Derivative liability fair value gross liability | 0.0 | 0.0
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Derivative asset fair value gross asset | 0.0 | 0.0
Derivative liability fair value gross liability | $ 0.0 | $ 0.0

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