Note 9 - Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - USD ($) $ in Millions	Sep. 30, 2022	Sep. 30, 2021
Derivative Assets	$ 116.2	$ 58.1
Derivative Liabilities	61.5	91.7
Fair Value, Recurring [Member]
Effects of netting, asset	(153.6)	(208.4)
Derivative Assets	164.6	108.1
Effects of netting, liability	(117.0)	(73.0)
Derivative Liabilities	61.5	91.7
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	318.2	166.6
Derivative Liability Fair Value Gross Liability	178.5	6.0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.0	149.9
Derivative Liability Fair Value Gross Liability	0.0	158.7
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	19.1	23.8
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	29.3	26.2
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	19.1	23.8
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	29.3	26.2
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
N Y M E X I C E Natural Gas Contracts [Member] \| Fair Value, Recurring [Member] \| Gas Utility [Member]
Effects of netting, asset	(57.8)	(104.0)
Derivative Assets	0.0	0.0
Effects of netting, liability	(30.7)	(0.3)
Derivative Liabilities	0.0	0.0
N Y M E X I C E Natural Gas Contracts [Member] \| Fair Value, Recurring [Member] \| Gas Marketing [Member]
Effects of netting, asset	(91.8)	(93.7)
Derivative Assets	0.0	21.0
Effects of netting, liability	(82.3)	(62.0)
Derivative Liabilities	0.0	0.0
N Y M E X 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	57.8	104.0
Derivative Liability Fair Value Gross Liability	30.7	0.3
N Y M E X 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	91.8	0.0
Derivative Liability Fair Value Gross Liability	82.3	0.0
N Y M E X 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 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	114.7
Derivative Liability Fair Value Gross Liability	0.0	62.0
N Y M E X 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 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	(4.0)	(5.5)
Derivative Assets	52.6	29.7
Effects of netting, liability	(4.0)	(5.5)
Derivative Liabilities	61.5	91.2
Natural Gas Commodity Contracts [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	56.6	0.0
Derivative Liability Fair Value Gross Liability	65.5	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	0.0	35.2
Derivative Liability Fair Value Gross Liability	0.0	96.7
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
Derivative Assets	63.6
Effects of netting, liability	(5.2)	(5.2)
Derivative Liabilities	7.4	0.5
Interest Rate Swap [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Other Segments [Member]
Derivative Asset Fair Value Gross Asset	63.6
Derivative Liability Fair Value Gross Liability	12.6	5.7
Interest Rate Swap [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Other Segments [Member]
Derivative Asset Fair Value Gross Asset	0.0
Derivative Liability Fair Value Gross Liability	0.0	0.0
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
Derivative Liability Fair Value Gross Liability	0.0	0.0
Spire Missouri [Member]
Derivative Assets	0.0	0.0
Derivative Liabilities	0.0	0.0
Spire Missouri [Member] \| Fair Value, Recurring [Member]
Effects of netting, asset	(57.8)	(104.0)
Derivative Assets	19.1	23.8
Spire Missouri [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	76.9	127.8
Spire Missouri [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.0	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	19.1	23.8
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	19.1	23.8
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] \| N Y M E X I C E Natural Gas Contracts [Member] \| Fair Value, Recurring [Member]
Effects of netting, asset	(57.8)	(104.0)
Derivative Assets	0.0	0.0
Effects of netting, liability	(30.7)	(0.3)
Derivative Liabilities	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 1 [Member]
Derivative Asset Fair Value Gross Asset	57.8	104.0
Derivative Liability Fair Value Gross Liability	30.7	0.3
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

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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