Note 8 - Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - Fair Value, Recurring [Member] - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023	Mar. 31, 2023
Derivative Asset Fair Value Gross Asset	$ 146.3
Effects of netting, asset	(15.0)
Derivative Liability Fair Value Gross Liability	22.7
Effects of netting, liability	(69.0)
Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	70.0
Derivative Liability Fair Value Gross Liability	66.4
Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	91.3
Derivative Liability Fair Value Gross Liability	25.3
U S Stock And Bond Mutual Funds [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	22.6
Effects of netting, asset	0.0
U S Stock And Bond Mutual Funds [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	16.6
Effects of netting, asset	0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	22.6
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	16.6
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	0.0
Effects of netting, asset	(3.7)
Derivative Liability Fair Value Gross Liability	0.0
Effects of netting, liability	(35.7)
N Y M E X And I C E Natural Gas Contracts [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	0.0
Effects of netting, asset	(8.6)
Derivative Liability Fair Value Gross Liability	0.0
Effects of netting, liability	(30.7)
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	3.7
Derivative Liability Fair Value Gross Liability	35.7
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	8.6
Derivative Liability Fair Value Gross Liability	30.7
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	0.0
Derivative Liability Fair Value Gross Liability	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	0.0
Derivative Liability Fair Value Gross Liability	0.0
Gasoline And Heating Oil Contracts [Member] | Gas Utility [Member]
Derivative Liability Fair Value Gross Liability	0.0
Effects of netting, liability	(0.1)
Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Derivative Liability Fair Value Gross Liability	0.0
Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]
Derivative Liability Fair Value Gross Liability	0.1
Natural Gas Commodity Contracts [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	59.6
Effects of netting, asset	(2.6)
Derivative Liability Fair Value Gross Liability	22.7
Effects of netting, liability	(2.6)
Natural Gas Commodity Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	0.0
Derivative Liability Fair Value Gross Liability	0.0
Natural Gas Commodity Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	62.2
Derivative Liability Fair Value Gross Liability	25.3
U.S. Bonds [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	17.1
Effects of netting, asset	0.0
U.S. Bonds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	17.1
U.S. Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	0.0
Interest Rate Swap [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	29.0
Effects of netting, asset	0.0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	0.0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	29.0
Global Bonds [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	1.4
Effects of netting, asset	0.0
Global Bonds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	1.4
Global Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset	0.0
Spire Missouri [Member]
Derivative Asset Fair Value Gross Asset	22.6	$ 20.4	$ 20.7
Effects of netting, asset	(3.8)	(6.4)	(0.1)
Derivative Liability Fair Value Gross Liability			0.0
Effects of netting, liability			(63.2)
Spire Missouri [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	26.3	26.7	20.8
Derivative Liability Fair Value Gross Liability			63.2
Spire Missouri [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.1	0.1	0.0
Derivative Liability Fair Value Gross Liability			0.0
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member]
Derivative Asset Fair Value Gross Asset	22.6	20.4	20.7
Effects of netting, asset	0.0	0.0	0.0
Derivative Liability Fair Value Gross Liability			0.0
Effects of netting, liability			(62.7)
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	22.6	20.4	20.7
Derivative Liability Fair Value Gross Liability			62.7
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0	0.0
Derivative Liability Fair Value Gross Liability			0.0
Spire Missouri [Member] | N Y M E X I C E Natural Gas Contracts [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0	0.0
Effects of netting, asset	(3.7)	(6.3)	(0.1)
Derivative Liability Fair Value Gross Liability	0.0	6.8
Effects of netting, liability	(35.7)	(44.0)
Spire Missouri [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	3.7	6.3	0.1
Derivative Liability Fair Value Gross Liability	35.7	50.8
Spire Missouri [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0	0.0
Derivative Liability Fair Value Gross Liability	0.0	0.0
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member]
Derivative Asset Fair Value Gross Asset		0.0
Effects of netting, asset	(0.1)	(0.1)
Derivative Liability Fair Value Gross Liability	0.0		0.0
Effects of netting, liability			(0.5)
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset		0.0
Derivative Liability Fair Value Gross Liability	0.0		0.5
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset		0.1
Derivative Liability Fair Value Gross Liability	$ 0.1		0.0
Spire Alabama Inc [Member]
Derivative Asset Fair Value Gross Asset		135.3	104.3
Effects of netting, asset		(20.1)	(33.3)
Derivative Liability Fair Value Gross Liability		30.8	33.3
Effects of netting, liability		(69.4)	(112.6)
Spire Alabama Inc [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset		74.4	81.9
Derivative Liability Fair Value Gross Liability		72.6	111.3
Spire Alabama Inc [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset		81.0	55.7
Derivative Liability Fair Value Gross Liability		27.6	34.6
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset		20.4	20.7
Effects of netting, asset		0.0	0.0
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset			0.0
Effects of netting, asset			(31.9)
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset		37.6	29.2
Effects of netting, asset		0.0	0.0
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset		20.4	20.7
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset			31.9
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset		37.6	29.2
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset		0.0	0.0
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset			0.0
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset		0.0	0.0
Spire Alabama Inc [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset		0.0	0.0
Effects of netting, asset		(6.3)	(0.1)
Derivative Liability Fair Value Gross Liability		6.8	0.0
Effects of netting, liability		(44.0)	(62.7)
Spire Alabama Inc [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset		0.0
Effects of netting, asset		(10.1)
Derivative Liability Fair Value Gross Liability		0.0	0.0
Effects of netting, liability		(21.8)	(48.1)
Spire Alabama Inc [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset		6.3	0.1
Derivative Liability Fair Value Gross Liability		50.8	62.7
Spire Alabama Inc [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset		10.1
Derivative Liability Fair Value Gross Liability		21.8	48.1
Spire Alabama Inc [Member] | N Y M E X I C E Natural Gas Contracts [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
Spire Alabama Inc [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset		0.0
Derivative Liability Fair Value Gross Liability		0.0	0.0
Spire Alabama Inc [Member] | Gasoline And Heating Oil Contracts [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset		0.0	0.0
Effects of netting, asset		(0.1)	(0.5)
Spire Alabama Inc [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset		0.0	0.5
Spire Alabama Inc [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset		0.1	0.0
Spire Alabama Inc [Member] | Natural Gas Commodity Contracts [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset		33.1	37.9
Effects of netting, asset		(3.6)	(1.3)
Derivative Liability Fair Value Gross Liability		24.0	31.1
Effects of netting, liability		(3.6)	(1.3)
Spire Alabama Inc [Member] | Natural Gas Commodity Contracts [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
Spire Alabama Inc [Member] | Natural Gas Commodity Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset		36.7	39.2
Derivative Liability Fair Value Gross Liability		27.6	32.4
Spire Alabama Inc [Member] | Interest Rate Swap [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset		44.2	16.5	[1]
Effects of netting, asset		0.0	0.0
Derivative Liability Fair Value Gross Liability			2.2
Effects of netting, liability			0.0
Spire Alabama Inc [Member] | Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset		0.0	0.0	[1]
Derivative Liability Fair Value Gross Liability			0.0
Spire Alabama Inc [Member] | Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Derivative Asset Fair Value Gross Asset		$ 44.2	16.5	[1]
Derivative Liability Fair Value Gross Liability			$ 2.2

[1] Subsequent to the issuance of its consolidated financial statements for the quarter ended March 31, 2023, during the fourth quarter of fiscal 2023, the Company identified an error in the fair value level presentation for certain line items in the Fair Value Measurements table. The presentaion has been corrected to reflect the impacted line items in Level 2 rather than Level 1 as of March 31, 2023. This immaterial correction did not impact the reported fair values or the consolidated financial statements.