Note 8 - Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - Fair Value, Recurring [Member] - USD ($) $ in Millions	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Derivative Asset Fair Value Gross Asset	$ 126.4
Effects of netting, asset	(19.3)
Derivative Liability Fair Value Gross Liability	25.1
Effects of netting, liability	(95.2)
Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	77.5
Derivative Liability Fair Value Gross Liability	93.4
Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	68.2
Derivative Liability Fair Value Gross Liability	26.9
U S Stock And Bond Mutual Funds [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	22.0
Effects of netting, asset	0.0
U S Stock And Bond Mutual Funds [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	14.6
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	22.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	14.6
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 Segments [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	(4.7)
Derivative Liability Fair Value Gross Liability	0.0
Effects of netting, liability	(60.7)
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	(13.2)
Derivative Liability Fair Value Gross Liability	0.0
Effects of netting, liability	(32.7)
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	4.7
Derivative Liability Fair Value Gross Liability	60.7
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	13.2
Derivative Liability Fair Value Gross Liability	32.7
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	(0.4)
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	0.4
Natural Gas Commodity Contracts [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	44.1
Effects of netting, asset	(1.4)
Derivative Liability Fair Value Gross Liability	23.1
Effects of netting, liability	(1.4)
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	45.5
Derivative Liability Fair Value Gross Liability	24.5
U.S. Bonds [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	21.6
Effects of netting, asset	0.0
U.S. Bonds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	21.6
U.S. Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	0.0
Global Bonds [Member] | Other Segments [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 Segments [Member]
Derivative Asset Fair Value Gross Asset	1.4
Global Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	0.0
Interest Rate Swap [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	22.7
Effects of netting, asset	0.0
Derivative Liability Fair Value Gross Liability	2.0
Effects of netting, liability	0.0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	0.0
Derivative Liability Fair Value Gross Liability	0.0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	22.7
Derivative Liability Fair Value Gross Liability	2.0
Spire Missouri [Member]
Derivative Asset Fair Value Gross Asset	22.0	$ 20.4	$ 19.5
Effects of netting, asset	(4.7)	(6.4)	(0.6)
Derivative Liability Fair Value Gross Liability	0.0		0.0
Effects of netting, liability	(61.1)		(35.9)
Spire Missouri [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	26.7	26.7	20.1
Derivative Liability Fair Value Gross Liability	60.7		35.9
Spire Missouri [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.1	0.0
Derivative Liability Fair Value Gross Liability	0.4		0.0
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member]
Derivative Asset Fair Value Gross Asset	22.0	20.4	19.5
Effects of netting, asset	0.0	0.0	0.0
Derivative Liability Fair Value Gross Liability			0.0
Effects of netting, liability			(35.7)
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	22.0	20.4	19.5
Derivative Liability Fair Value Gross Liability			35.7
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	(4.7)	(6.3)	(0.4)
Derivative Liability Fair Value Gross Liability	0.0	6.8
Effects of netting, liability	(60.7)	(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	4.7	6.3	0.4
Derivative Liability Fair Value Gross Liability	60.7	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	0.0
Effects of netting, asset		(0.1)	(0.2)
Derivative Liability Fair Value Gross Liability	0.0		0.0
Effects of netting, liability	(0.4)		(0.2)
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset		0.0	0.2
Derivative Liability Fair Value Gross Liability	0.0		0.2
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset		$ 0.1	0.0
Derivative Liability Fair Value Gross Liability	0.4		0.0
Spire Alabama Inc [Member]
Derivative Asset Fair Value Gross Asset	135.3		188.9
Effects of netting, asset	(20.1)		(65.8)
Derivative Liability Fair Value Gross Liability	30.8		58.7
Effects of netting, liability	(69.4)		(107.5)
Spire Alabama Inc [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	74.4		100.4
Derivative Liability Fair Value Gross Liability	72.6		94.3
Spire Alabama Inc [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	81.0		154.3
Derivative Liability Fair Value Gross Liability	27.6		71.9
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	20.4		19.5
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			(52.0)
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	37.6		28.3
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		19.5
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			52.0
Spire Alabama Inc [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	37.6		28.3
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 Segments [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)		(0.4)
Derivative Liability Fair Value Gross Liability	6.8		0.0
Effects of netting, liability	(44.0)		(35.7)
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)		(58.4)
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		0.4
Derivative Liability Fair Value Gross Liability	50.8		35.7
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		58.4
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)		(0.2)
Derivative Liability Fair Value Gross Liability			0.0
Effects of netting, liability			(0.2)
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		0.2
Derivative Liability Fair Value Gross Liability			0.2
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
Derivative Liability Fair Value Gross Liability			0.0
Spire Alabama Inc [Member] | Natural Gas Commodity Contracts [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	33.1		79.7
Effects of netting, asset	(3.6)		(13.2)
Derivative Liability Fair Value Gross Liability	24.0		57.9
Effects of netting, liability	(3.6)		(13.2)
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		92.9
Derivative Liability Fair Value Gross Liability	27.6		71.1
Spire Alabama Inc [Member] | Interest Rate Swap [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	44.2		61.4	[1]
Effects of netting, asset	0.0		0.0
Derivative Liability Fair Value Gross Liability			0.8
Effects of netting, liability			0.0
Spire Alabama Inc [Member] | Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member] | Other Segments [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 Segments [Member]
Derivative Asset Fair Value Gross Asset	$ 44.2		61.4	[1]
Derivative Liability Fair Value Gross Liability			$ 0.8

[1] Subsequent to the issuance of its consolidated financial statements for the quarter ended December 31, 2022, during the fourth quarter of fiscal 2023, the Company iden??fied an error in the fair value level presenta??on for certain line items in the Fair Value Measurements table. The presenta??on has been corrected to reflect the impacted line items in Level 2 rather than Level 1 as of December 31, 2022. This immaterial correc??on did not impact the reported fair values or the consolidated financial statements.