v3.25.2
Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - Fair Value, Recurring [Member] - USD ($)
$ in Millions
Jun. 30, 2025
Sep. 30, 2024
Jun. 30, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset $ (15.8) $ (13.9) $ (13.7)
Derivative Assets 136.4 113.3 154.3
Effects of netting, liability (21.2) (25.0) (46.4)
Derivative Liabilities 19.4 32.3 27.0
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 94.0 80.4 81.9
Derivative liability fair value gross liability 24.2 31.3 43.6
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 58.2 46.8 86.1
Derivative liability fair value gross liability 16.4 26.0 29.8
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 26.3 24.3 24.5
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 31.5 17.9 30.7
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 26.3 24.3 24.5
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 31.5 17.9 30.7
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 (5.1) (3.4) (4.8)
Derivative Assets 0.0 0.0 0.0
Effects of netting, liability (14.0) (3.8) (21.0)
Derivative Liabilities 0.0 9.8 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 (8.6) (7.0) (6.1)
Derivative Assets 0.0 0.0 0.0
Effects of netting, liability (5.1) (17.7) (22.6)
Derivative Liabilities 5.1 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 5.1 3.4 4.8
Derivative liability fair value gross liability 14.0 13.6 21.0
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 8.6 7.0 6.1
Derivative liability fair value gross liability 10.2 17.7 22.6
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 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
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 (2.1) (3.5) (2.8)
Derivative Assets 44.5 42.5 51.1
Effects of netting, liability (2.1) (3.5) (2.8)
Derivative Liabilities 14.3 21.0 27.0
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 46.6 46.0 53.9
Derivative liability fair value gross liability 16.4 24.5 29.8
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 18.0 21.9 14.4
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 18.0 21.9 14.4
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 4.5 5.9 1.4
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 4.5 5.9 1.4
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 11.6 0.8 32.2
Effects of netting, liability   0.0  
Derivative Liabilities   1.5  
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
Derivative liability fair value gross liability   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 11.6 0.8 32.2
Derivative liability fair value gross liability   1.5  
Spire Missouri [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Effects of netting, asset (5.1) (3.4) (4.8)
Derivative Assets 26.3 24.3 24.5
Effects of netting, liability (5.1)    
Derivative Liabilities 5.1    
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 31.4 27.7 29.3
Derivative liability fair value gross liability 10.2    
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
Derivative liability fair value gross liability 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 26.3 24.3 24.5
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 26.3 24.3 24.5
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 (5.1) (3.4) (4.8)
Derivative Assets 0.0 0.0 0.0
Effects of netting, liability (5.1) (3.8) (21.0)
Derivative Liabilities 5.1 9.8 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 5.1 3.4 4.8
Derivative liability fair value gross liability 10.2 13.6 21.0
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