Note 9 - Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Derivative Assets	$ 77.3	$ 116.2
Derivative Liabilities	30.8	61.5
Fair Value, Recurring [Member]
Effects of netting, asset	(20.1)	(153.6)
Derivative Assets	135.3	164.6
Effects of netting, liability	(69.4)	(117.0)
Derivative Liabilities	30.8	61.5
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	74.4	198.0
Derivative Liability Fair Value Gross Liability	72.6	113.0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	81.0	120.2
Derivative Liability Fair Value Gross Liability	27.6	65.5
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
Derivative Assets	20.4	19.1
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Other Segments [Member]
Effects of netting, asset	0.0	0.0
Derivative Assets	37.6	29.3
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	20.4	19.1
U S Stock And Bond Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	37.6	29.3
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 Segments [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 Segments [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0
Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Gas Utility [Member]
Effects of netting, asset	(0.1)
Derivative Assets	0.0
Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	0.0
Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	0.1
Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	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	(6.3)	(57.8)
Derivative Assets	0.0	0.0
Effects of netting, liability	(44.0)	(30.7)
Derivative Liabilities	6.8	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Gas Marketing [Member]
Effects of netting, asset	(10.1)	(91.8)
Derivative Assets	0.0	0.0
Effects of netting, liability	(21.8)	(82.3)
Derivative Liabilities	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	6.3	57.8
Derivative Liability Fair Value Gross Liability	50.8	30.7
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	10.1	91.8
Derivative Liability Fair Value Gross Liability	21.8	82.3
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	(3.6)	(4.0)
Derivative Assets	33.1	52.6
Effects of netting, liability	(3.6)	(4.0)
Derivative Liabilities	24.0	61.5
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	36.7	56.6
Derivative Liability Fair Value Gross Liability	27.6	65.5
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
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Other Segments [Member]
Effects of netting, asset	0.0	0.0
Derivative Assets	44.2	63.6
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	44.2	63.6
Interest Rate Swap [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0
Spire Missouri [Member]
Derivative Assets	0.0	0.0
Derivative Liabilities	6.8	0.0
Spire Missouri [Member] | Fair Value, Recurring [Member]
Effects of netting, asset	(6.4)	(57.8)
Derivative Assets	20.4	19.1
Spire Missouri [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	26.7	76.9
Spire Missouri [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.1	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
Derivative Assets	20.4	19.1
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	20.4	19.1
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] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member]
Effects of netting, asset	(0.1)
Derivative Assets	0.0
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	0.0
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.1
Spire Missouri [Member] | Gasoline And Heating Oil Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative Asset Fair Value Gross Asset	0.0
Spire Missouri [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member]
Effects of netting, asset	(6.3)	(57.8)
Derivative Assets	0.0	0.0
Effects of netting, liability	(44.0)	(30.7)
Derivative Liabilities	6.8	0.0
Spire Missouri [Member] | N Y M E X I C E Natural Gas Contracts [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	6.3	57.8
Derivative Liability Fair Value Gross Liability	50.8	30.7
Spire Missouri [Member] | N Y M E X 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 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