Note 8 - Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - Fair Value, Recurring [Member] - USD ($) $ in Millions	Jun. 30, 2023	Sep. 30, 2022	Jun. 30, 2022
Effects of netting, asset	$ (11.5)	$ (153.6)	$ (146.7)
Derivative Assets	110.4	164.6	149.3
Effects of netting, liability	(70.8)	(117.0)	(86.5)
Derivative Liabilities	34.7	61.5	113.0
Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	99.1	254.6	243.3
Derivative Liability Fair Value Gross Liability	104.7	178.5	191.9
Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	22.8	63.6	52.7
Derivative Liability Fair Value Gross Liability	0.8	0.0	7.6
Spire Missouri [Member]
Effects of netting, asset		(57.8)	(39.6)
Derivative Assets		19.1	20.3
Effects of netting, liability	(45.5)
Derivative Liabilities	0.0
Spire Missouri [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset		76.9	59.9
Derivative Liability Fair Value Gross Liability	45.5
Spire Missouri [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
U S Stock And Bond Mutual Funds [Member] | Gas Utility [Member]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	21.4	19.1	20.3
U S Stock And Bond Mutual Funds [Member] | Other Segments [Member]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	31.4	29.3	31.6
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	21.4	19.1	20.3
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	31.4	29.3	31.6
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	0.0	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	0.0	0.0
U S Stock And Bond Mutual Funds [Member] | Spire Missouri [Member]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	21.4	19.1	20.3
U S Stock And Bond Mutual Funds [Member] | Spire Missouri [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	21.4	19.1	20.3
U S Stock And Bond Mutual Funds [Member] | Spire Missouri [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0	0.0
N Y M E X I C E Natural Gas Contracts [Member] | Gas Utility [Member]
Effects of netting, asset		(57.8)	(39.6)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(44.8)	(30.7)	(12.8)
Derivative Liabilities		0.0	0.0
N Y M E X I C E Natural Gas Contracts [Member] | Gas Marketing [Member]
Effects of netting, asset	(10.5)	(91.8)	(94.2)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(24.3)	(82.3)	(60.8)
Derivative Liabilities	0.0	0.0	0.0
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		57.8	39.6
Derivative Liability Fair Value Gross Liability	44.8	30.7	12.8
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	10.5	91.8	94.2
Derivative Liability Fair Value Gross Liability	24.3	82.3	60.8
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	0.0
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	0.0	0.0
Derivative Liability Fair Value Gross Liability	0.0	0.0	0.0
N Y M E X I C E Natural Gas Contracts [Member] | Spire Missouri [Member]
Effects of netting, asset		(57.8)	(39.6)
Derivative Assets		0.0	0.0
Effects of netting, liability	(44.8)	(30.7)	(12.8)
Derivative Liabilities	0.0	0.0	0.0
N Y M E X I C E Natural Gas Contracts [Member] | Spire Missouri [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset		57.8	39.6
Derivative Liability Fair Value Gross Liability	44.8	30.7	12.8
N Y M E X I C E Natural Gas Contracts [Member] | Spire Missouri [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	0.0
Gasoline And Heating Oil Contracts [Member] | Gas Utility [Member]
Derivative Assets	0.0
Effects of netting, liability	(0.7)
Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Derivative Liability Fair Value Gross Liability	0.7
Gasoline And Heating Oil Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]
Derivative Liability Fair Value Gross Liability	0.0
Gasoline And Heating Oil Contracts [Member] | Spire Missouri [Member]
Effects of netting, liability	(0.7)
Derivative Liabilities	0.0
Gasoline And Heating Oil Contracts [Member] | Spire Missouri [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Liability Fair Value Gross Liability	0.7
Gasoline And Heating Oil Contracts [Member] | Spire Missouri [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Liability Fair Value Gross Liability	0.0
Natural Gas Commodity Contracts [Member] | Gas Marketing [Member]
Effects of netting, asset	(1.0)	(4.0)	(5.3)
Derivative Assets	34.8	52.6	52.3
Effects of netting, liability	(1.0)	(4.0)	(5.3)
Derivative Liabilities	33.9	61.5	113.0
Natural Gas Commodity Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	35.8	56.6	57.6
Derivative Liability Fair Value Gross Liability	34.9	65.5	118.3
Natural Gas Commodity Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0	0.0
Derivative Liability Fair Value Gross Liability	0.0	0.0	0.0
Interest Rate Swap [Member]
Effects of netting, asset			(7.6)
Derivative Assets			45.1
Interest Rate Swap [Member] | Other Segments [Member]
Effects of netting, asset	0.0	0.0
Derivative Assets	22.8	63.6
Effects of netting, liability	0.0		(7.6)
Derivative Liabilities	0.8		0.0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset			0.0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member] | Other Segments [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, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset			52.7
Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member] | Other Segments [Member]
Derivative Asset Fair Value Gross Asset	22.8	$ 63.6
Derivative Liability Fair Value Gross Liability	$ 0.8		$ 7.6