Note 7 - Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - Fair Value, Recurring [Member] - USD ($) $ in Millions	Jun. 30, 2022	Sep. 30, 2021	Jun. 30, 2021
Effects of netting, asset	$ (146.7)	$ (208.4)	$ (78.3)
Derivative Assets	149.3	108.1	69.5
Effects of netting, liability	(86.5)	(73.0)	(21.5)
Derivative Liabilities	113.0	91.7	61.8
Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	144.2	166.6	71.1
Derivative Liability Fair Value Gross Liability	20.4	6.0	0.0
Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	151.8	149.9	76.7
Derivative Liability Fair Value Gross Liability	179.1	158.7	83.3
Spire Missouri [Member]
Effects of netting, asset	(39.6)	(104.0)	(25.3)
Derivative Assets	20.3	23.8	24.3
Spire Missouri [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	59.9	127.8	49.6
Spire Missouri [Member] \| Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0	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	20.3	23.8	24.3
U S Stock And Bond Mutual Funds [Member] \| Other Segments [Member]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	31.6	26.2	21.5
U S Stock And Bond Mutual Funds [Member] \| Fair Value, Inputs, Level 1 [Member] \| Gas Utility [Member]
Derivative Asset Fair Value Gross Asset	20.3	23.8	24.3
U S Stock And Bond Mutual Funds [Member] \| Fair Value, Inputs, Level 1 [Member] \| Other Segments [Member]
Derivative Asset Fair Value Gross Asset	31.6	26.2	21.5
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	20.3	23.8	24.3
U S Stock And Bond Mutual Funds [Member] \| Spire Missouri [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	20.3	23.8	24.3
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	(39.6)	(104.0)	(25.3)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(12.8)	(0.3)	(16.0)
Derivative Liabilities	0.0	0.0	2.8
N Y M E X I C E Natural Gas Contracts [Member] \| Gas Marketing [Member]
Effects of netting, asset	(94.2)	(93.7)	(47.5)
Derivative Assets	0.0	21.0	0.0
Effects of netting, liability	(60.8)	(62.0)
Derivative Liabilities	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	39.6	104.0	25.3
Derivative Liability Fair Value Gross Liability	12.8	0.3	0.0
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	0.0	0.0	0.0
Derivative Liability Fair Value Gross Liability	0.0	0.0
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	0.0
Derivative Liability Fair Value Gross Liability	0.0	0.0	18.8
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	94.2	114.7	47.5
Derivative Liability Fair Value Gross Liability	60.8	62.0
N Y M E X I C E Natural Gas Contracts [Member] \| Spire Missouri [Member]
Effects of netting, asset	(39.6)	(104.0)	(25.3)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(12.8)	(0.3)
Derivative Liabilities	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	39.6	104.0	25.3
Derivative Liability Fair Value Gross Liability	12.8	0.3
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	0.0
Derivative Liability Fair Value Gross Liability	0.0	0.0
Natural Gas Commodity Contracts [Member] \| Gas Marketing [Member]
Effects of netting, asset	(5.3)	(5.5)	0.0
Derivative Assets	52.3	29.7	20.2
Effects of netting, liability	(5.3)	(5.5)	0.0
Derivative Liabilities	113.0	91.2	57.8
Natural Gas Commodity Contracts [Member] \| Fair Value, Inputs, Level 1 [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
Natural Gas Commodity Contracts [Member] \| Fair Value, Inputs, Level 2 [Member] \| Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	57.6	35.2	20.2
Derivative Liability Fair Value Gross Liability	118.3	96.7	57.8
Interest Rate Swap [Member] \| Other Segments [Member]
Effects of netting, asset	(7.6)	(5.2)	(5.5)
Derivative Assets	45.1	7.4	3.5
Effects of netting, liability	(7.6)	(5.2)	(5.5)
Derivative Liabilities	0.0	0.5	1.2
Interest Rate Swap [Member] \| Fair Value, Inputs, Level 1 [Member] \| Other Segments [Member]
Derivative Asset Fair Value Gross Asset	52.7	12.6	0.0
Derivative Liability Fair Value Gross Liability	7.6	5.7	0.0
Interest Rate Swap [Member] \| Fair Value, Inputs, Level 2 [Member] \| Other Segments [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0	9.0
Derivative Liability Fair Value Gross Liability	$ 0.0	$ 0.0	$ 6.7

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset.

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Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and obligation to return cash collateral under master netting arrangements.

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