Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - Fair Value, Recurring [Member] - USD ($) $ in Millions	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	$ (15.9)	$ (13.9)	$ (19.3)
Derivative Assets	126.1	113.3	126.4
Effects of netting, liability	(23.1)	(25.0)	(95.2)
Derivative Liabilities	27.9	32.3	25.1
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	81.8	80.4	77.5
Derivative liability fair value gross liability	21.9	31.3	93.4
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	60.2	46.8	68.2
Derivative liability fair value gross liability	29.1	26.0	26.9
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	24.7	24.3	22.0
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	20.8	17.9	14.6
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	24.7	24.3	22.0
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	20.8	17.9	14.6
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	(5.9)	(3.4)	(4.7)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(6.8)	(3.8)	(60.7)
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	(8.8)	(7.0)	(13.2)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(15.1)	(17.7)	(32.7)
Derivative Liabilities	0.0	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	5.9	3.4	4.7
Derivative liability fair value gross liability	6.8	13.6	60.7
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	8.8	7.0	13.2
Derivative liability fair value gross liability	15.1	17.7	32.7
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
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, liability			(0.4)
Derivative Liabilities			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 liability fair value gross liability			0.0
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 liability fair value gross liability			0.4
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	(1.2)	(3.5)	(1.4)
Derivative Assets	42.9	42.5	44.1
Effects of netting, liability	(1.2)	(3.5)	(1.4)
Derivative Liabilities	27.9	21.0	23.1
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	44.1	46.0	45.5
Derivative liability fair value gross liability	29.1	24.5	24.5
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	15.6	21.9	21.6
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	15.6	21.9	21.6
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	6.0	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	6.0	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] | Gas Marketing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, liability		0.0
Derivative Liabilities		1.5
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	16.1	0.8	22.7
Effects of netting, liability			0.0
Derivative Liabilities			2.0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability fair value gross liability		0.0
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] | Gas Marketing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability fair value gross liability		1.5
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	16.1	0.8	22.7
Derivative liability fair value gross liability			2.0
Spire Missouri [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	(5.9)	(3.4)	(4.7)
Derivative Assets	24.7	24.3	22.0
Effects of netting, liability			(61.1)
Derivative Liabilities			0.0
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	30.6	27.7	26.7
Derivative liability fair value gross liability			60.7
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			0.4
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	24.7	24.3	22.0
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	24.7	24.3	22.0
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	(5.9)	(3.4)	(4.7)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(6.8)	(3.8)	(60.7)
Derivative Liabilities	0.0	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	5.9	3.4	4.7
Derivative liability fair value gross liability	6.8	13.6	60.7
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
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, liability			(0.4)
Derivative Liabilities			0.0
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability fair value gross liability			0.0
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability fair value gross liability			$ 0.4