Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Total	$ 52.1	$ 43.3
Total	38.6	32.3
Fair Value, Recurring [Member]
Effects of netting, asset	(17.0)	(13.9)
Total	$ 131.6	$ 113.3
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Other Investments	Other Investments
Effects of netting, liability	$ (23.0)	$ (25.0)
Total	$ 38.6	$ 32.3
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Other libilities	Other libilities
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative asset fair value gross asset	$ 93.8	$ 80.4
Derivative liability fair value gross liability	44.9	31.3
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative asset fair value gross asset	54.8	46.8
Derivative liability fair value gross liability	16.7	26.0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative asset fair value gross asset	0.0	0.0
Derivative liability fair value gross liability	0.0	0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Gas Utility [Member]
Effects of netting, asset	0.0	0.0
Total	27.5	24.3
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Other Operating Segment [Member]
Effects of netting, asset	0.0	0.0
Total	27.0	17.9
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Derivative asset fair value gross asset	27.5	24.3
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	27.0	17.9
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]
Derivative asset fair value gross asset	0.0	0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	0.0	0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Utility [Member]
Derivative asset fair value gross asset	0.0	0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	0.0	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Gas Utility [Member]
Effects of netting, asset	(2.9)	(3.4)
Total	0.0	0.0
Effects of netting, liability	(1.3)	(3.8)
Total	24.6	9.8
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Gas Marketing [Member]
Effects of netting, asset	(11.4)	(7.0)
Total	0.0	0.0
Effects of netting, liability	(19.0)	(17.7)
Total	0.0	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Derivative asset fair value gross asset	2.9	3.4
Derivative liability fair value gross liability	25.9	13.6
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]
Derivative asset fair value gross asset	11.4	7.0
Derivative liability fair value gross liability	19.0	17.7
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]
Derivative asset fair value gross asset	0.0	0.0
Derivative liability fair value gross liability	0.0	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]
Derivative asset fair value gross asset	0.0	0.0
Derivative liability fair value gross liability	0.0	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Utility [Member]
Derivative asset fair value gross asset	0.0	0.0
Derivative liability fair value gross liability	0.0	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Marketing [Member]
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, Recurring [Member] | Gas Marketing [Member]
Effects of netting, asset	(2.7)	(3.5)
Total	42.1	42.5
Effects of netting, liability	(2.7)	(3.5)
Total	14.0	21.0
Natural Gas Commodity Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]
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, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]
Derivative asset fair value gross asset	44.8	46.0
Derivative liability fair value gross liability	16.7	24.5
Natural Gas Commodity Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Marketing [Member]
Derivative asset fair value gross asset	0.0	0.0
Derivative liability fair value gross liability	0.0	0.0
U.S. Bonds [Member] | Fair Value, Recurring [Member] | Other Operating Segment [Member]
Total	23.5	21.9
U.S. Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	23.5	21.9
U.S. Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	0.0	0.0
U.S. Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	0.0	0.0
Global Bonds [Member] | Fair Value, Recurring [Member] | Other Operating Segment [Member]
Effects of netting, asset	0.0	0.0
Total	1.5	5.9
Global Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	1.5	5.9
Global Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	0.0	0.0
Global Bonds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	0.0	0.0
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Other Operating Segment [Member]
Effects of netting, asset	0.0	0.0
Total	10.0	0.8
Effects of netting, liability		0.0
Total		1.5
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	0.0	0.0
Derivative liability fair value gross liability		0.0
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	10.0	0.8
Derivative liability fair value gross liability		1.5
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Operating Segment [Member]
Derivative asset fair value gross asset	0.0	0.0
Derivative liability fair value gross liability		0.0
Spire Missouri [Member]
Total	0.0	0.0
Total	24.6	9.8
Spire Missouri [Member] | Fair Value, Recurring [Member]
Effects of netting, asset	(2.9)	(3.4)
Total	$ 27.5	$ 24.3
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Other Property and Investments	Other Property and Investments
Spire Missouri [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative asset fair value gross asset	$ 30.4	$ 27.7
Spire Missouri [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative asset fair value gross asset	0.0	0.0
Spire Missouri [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative asset fair value gross asset	0.0	0.0
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member]
Effects of netting, asset	0.0	0.0
Total	27.5	24.3
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative asset fair value gross asset	27.5	24.3
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative asset fair value gross asset	0.0	0.0
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative asset fair value gross asset	0.0	0.0
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member]
Effects of netting, asset	(2.9)	(3.4)
Total	0.0	0.0
Effects of netting, liability	(1.3)	(3.8)
Total	24.6	9.8
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative asset fair value gross asset	2.9	3.4
Derivative liability fair value gross liability	25.9	13.6
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative asset fair value gross asset	0.0	0.0
Derivative liability fair value gross liability	0.0	0.0
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative asset fair value gross asset	0.0	0.0
Derivative liability fair value gross liability	$ 0.0	$ 0.0