v3.26.1
Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - Fair Value, Recurring [Member] - USD ($)
$ in Millions
Jun. 30, 2026
Sep. 30, 2025
Jun. 30, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset $ (2.3) $ (17.0) $ (15.8)
Derivative Assets $ 90.7 $ 131.6 $ 136.4
Derivative Asset, Statement of Financial Position [Extensible Enumeration] Other Investments Other Investments Other Investments
Effects of netting, liability $ (2.3) $ (23.0) $ (21.2)
Derivative Liabilities $ 14.6 $ 38.6 $ 19.4
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 $ 76.7 $ 93.8 $ 94.0
Derivative liability fair value gross liability 14.3 44.9 24.2
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 16.3 54.8 58.2
Derivative liability fair value gross liability 2.6 16.7 16.4
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 29.0 27.5 26.3
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 19.3 27.0 31.5
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 29.0 27.5 26.3
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 19.3 27.0 31.5
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 (2.3) (2.9)  
Derivative Assets 0.0 0.0  
Effects of netting, liability (2.3) (1.3) (5.1)
Derivative Liabilities 12.0 24.6 5.1
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   (11.4) (8.6)
Derivative Assets   0.0 0.0
Effects of netting, liability   (19.0) (14.0)
Derivative Liabilities   0.0 0.0
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 2.3 2.9  
Derivative liability fair value gross liability 14.3 25.9 10.2
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   11.4 8.6
Derivative liability fair value gross liability   19.0 14.0
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
Derivative liability fair value gross liability   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     (5.1)
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     5.1
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   (2.7) (2.1)
Derivative Assets   42.1 44.5
Effects of netting, liability   (2.7) (2.1)
Derivative Liabilities   14.0 14.3
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
Derivative liability fair value gross liability   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   44.8 46.6
Derivative liability fair value gross liability   16.7 16.4
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 25.1 23.5 18.0
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 25.1 23.5 18.0
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
Derivative Assets   10.0 11.6
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
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   10.0 11.6
Interest Rate Derivatives [Member] | Other Operating Segment [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset 0.0    
Derivative Assets 16.3    
Effects of netting, liability 0.0    
Derivative Liabilities 2.6    
Interest Rate Derivatives [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    
Derivative liability fair value gross liability 0.0    
Interest Rate Derivatives [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 16.3    
Derivative liability fair value gross liability 2.6    
Spire Missouri [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset   (2.9) (5.1)
Derivative Assets $ 29.0 $ 27.5 $ 26.3
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 $ 31.3 $ 30.4 $ 31.4
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 29.0 27.5 26.3
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 29.0 27.5 26.3
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 (2.3) (2.9) (5.1)
Derivative Assets 0.0 0.0 0.0
Effects of netting, liability (2.3) (1.3) (5.1)
Derivative Liabilities 12.0 24.6 5.1
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 2.3 2.9 5.1
Derivative liability fair value gross liability 14.3 25.9 10.2
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