Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - Fair Value, Recurring [Member] - USD ($) $ in Millions	Mar. 31, 2025	Sep. 30, 2024	Mar. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	$ (37.7)	$ (13.9)	$ (15.0)
Derivative Assets	122.3	113.3	146.3
Effects of netting, liability	(26.9)	(25.0)	(69.0)
Derivative Liabilities	39.2	32.3	22.7
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	100.8	80.4	70.0
Derivative liability fair value gross liability	23.4	31.3	66.4
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	59.2	46.8	91.3
Derivative liability fair value gross liability	42.7	26.0	25.3
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	24.6	24.3	22.6
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	22.0	17.9	16.6
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	24.6	24.3	22.6
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	22.0	17.9	16.6
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	(16.6)	(3.4)	(3.7)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(4.0)	(3.8)	(35.7)
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	(17.5)	(7.0)	(8.6)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(19.3)	(17.7)	(30.7)
Derivative Liabilities	0.1	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	16.6	3.4	3.7
Derivative liability fair value gross liability	4.0	13.6	35.7
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	17.5	7.0	8.6
Derivative liability fair value gross liability	19.4	17.7	30.7
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, asset			(0.1)
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			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 asset fair value gross asset			0.1
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	(3.6)	(3.5)	(2.6)
Derivative Assets	44.9	42.5	59.6
Effects of netting, liability	(3.6)	(3.5)	(2.6)
Derivative Liabilities	39.1	21.0	22.7
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	48.5	46.0	62.2
Derivative liability fair value gross liability	42.7	24.5	25.3
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	17.1
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	17.1
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	10.7	0.8	29.0
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	10.7	0.8	29.0
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	(17.5)	(3.4)	(3.8)
Derivative Assets	24.6	24.3	22.6
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	42.1	27.7	26.3
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	0.1
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	24.6	24.3	22.6
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	24.6	24.3	22.6
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	(17.5)	(3.4)	(3.7)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(4.0)	(3.8)	(35.7)
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	17.5	3.4	3.7
Derivative liability fair value gross liability	4.0	13.6	35.7
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
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, asset			(0.1)
Derivative Assets			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 asset fair value gross asset			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 asset fair value gross asset			$ 0.1