v3.26.1
Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - Fair Value, Recurring [Member] - USD ($)
$ in Millions
Mar. 31, 2026
Sep. 30, 2025
Mar. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset $ (12.5) $ (17.0) $ (37.7)
Derivative Assets $ 172.0 $ 131.6 $ 122.3
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Investments Other Investments Other Investments
Effects of netting, liability $ (21.7) $ (23.0) $ (26.9)
Derivative Liabilities $ 39.3 $ 38.6 $ 39.2
Derivative Liability, Statement of Financial Position [Extensible Enumeration] Other noncurrent liabilities Other noncurrent liabilities Other noncurrent liabilities
Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset $ 80.8 $ 93.8 $ 100.8
Derivative liability fair value gross liability 27.6 44.9 23.4
Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 103.7 54.8 59.2
Derivative liability fair value gross liability 33.4 16.7 42.7
U S Stock And Bond Mutual Funds [Member] | Gas Utility [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset 0.0 0.0 0.0
Derivative Assets 26.4 27.5 24.6
U S Stock And Bond Mutual Funds [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset 0.0 0.0 0.0
Derivative Assets 18.5 27.0 22.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 26.4 27.5 24.6
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 18.5 27.0 22.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 0.0 0.0 0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 0.0 0.0 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Gas Utility [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset (2.8) (2.9)  
Derivative Assets 0.0 0.0  
Effects of netting, liability (2.8) (1.3) (4.0)
Derivative Liabilities 10.6 24.6 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Gas Marketing [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset (5.0) (11.4) (16.6)
Derivative Assets 0.0 0.0 0.0
Effects of netting, liability (14.2) (19.0) (19.3)
Derivative Liabilities 0.0 0.0 0.1
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 2.8 2.9  
Derivative liability fair value gross liability 13.4 25.9 4.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 5.0 11.4 16.6
Derivative liability fair value gross liability 14.2 19.0 19.4
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 0.0 0.0  
Derivative liability fair value gross liability 0.0 0.0 0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 0.0 0.0 0.0
Derivative liability fair value gross liability 0.0 0.0 0.0
Gasoline And Heating Oil Contracts [Member] | Gas Utility [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset     (17.5)
Derivative Assets     0.0
Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset     17.5
Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset     0.0
Natural Gas Commodity Contracts [Member] | Gas Marketing [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset (4.7) (2.7) (3.6)
Derivative Assets 85.5 42.1 44.9
Effects of netting, liability (4.7) (2.7) (3.6)
Derivative Liabilities 28.7 14.0 39.1
Natural Gas Commodity Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 0.0 0.0 0.0
Derivative liability fair value gross liability 0.0 0.0 0.0
Natural Gas Commodity Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 90.2 44.8 48.5
Derivative liability fair value gross liability 33.4 16.7 42.7
U.S. Bonds [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset 0.0 0.0 0.0
Derivative Assets 27.1 23.5 15.6
U.S. Bonds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 27.1 23.5 15.6
U.S. Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 0.0 0.0 0.0
Global Bonds [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset 0.0 0.0 0.0
Derivative Assets 1.0 1.5 4.5
Global Bonds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 1.0 1.5 4.5
Global Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 0.0 0.0 0.0
Interest Rate Swap [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset 0.0 0.0 0.0
Derivative Assets 13.5 10.0 10.7
Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 0.0 0.0 0.0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 13.5 10.0 10.7
Spire Missouri [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset (2.8) (2.9) (17.5)
Derivative Assets $ 26.4 $ 27.5 $ 24.6
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Property and Investments Other Property and Investments Other Property and Investments
Spire Missouri [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset $ 29.2 $ 30.4 $ 42.1
Spire Missouri [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 0.0 0.0 0.0
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset 0.0 0.0 0.0
Derivative Assets 26.4 27.5 24.6
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 26.4 27.5 24.6
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 0.0 0.0 0.0
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset (2.8) (2.9) (17.5)
Derivative Assets 0.0 0.0 0.0
Effects of netting, liability (2.8) (1.3) (4.0)
Derivative Liabilities 10.6 24.6 0.0
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 2.8 2.9 17.5
Derivative liability fair value gross liability 13.4 25.9 4.0
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative asset fair value gross asset 0.0 0.0 0.0
Derivative liability fair value gross liability $ 0.0 $ 0.0 $ 0.0