Note 7 - Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - USD ($) $ in Millions	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Gasoline And Heating Oil Contracts [Member] \| Parent Company [Member] \| Term Loan [Member]
Derivative Asset Fair Value Gross Asset	$ 0.0
Derivative Liability Fair Value Gross Liability	0.0
Gasoline And Heating Oil Contracts [Member] \| Commercial Paper [Member] \| Parent Company [Member]
Derivative Asset Fair Value Gross Asset	0.2
Derivative Liability Fair Value Gross Liability	0.2
Spire Missouri [Member] \| Gasoline And Heating Oil Contracts [Member] \| Term Loan [Member]
Effects of netting, asset	(0.2)
Effects of netting, liability	(0.2)
Spire Missouri [Member] \| Gasoline And Heating Oil Contracts [Member] \| Spire Note [Member]
Derivative Assets	0.0
Derivative Liabilities	0.0
Fair Value, Recurring [Member]
Effects of netting, asset	(65.8)	$ (153.6)	$ (77.3)
Derivative Assets	188.9	164.6	91.3
Effects of netting, liability	(107.5)	(117.0)	(56.7)
Derivative Liabilities	58.7	61.5	77.3
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	254.7	318.2	61.2
Derivative Liability Fair Value Gross Liability	166.2	178.5	15.9
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0	107.4
Derivative Liability Fair Value Gross Liability	0.0	0.0	118.1
Fair Value, Recurring [Member] \| Spire Missouri [Member]
Effects of netting, asset	(0.6)	(57.8)	(2.4)
Derivative Assets	19.5	19.1	24.8
Effects of netting, liability	(35.9)
Derivative Liabilities	0.0
Fair Value, Recurring [Member] \| Spire Missouri [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	20.1	76.9	27.2
Derivative Liability Fair Value Gross Liability	35.9
Fair Value, Recurring [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
U S Stock And Bond Mutual Funds [Member] \| Fair Value, Recurring [Member] \| Gas Utility [Member]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	19.5	19.1	24.8
U S Stock And Bond Mutual Funds [Member] \| Fair Value, Recurring [Member] \| Other Segments [Member]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	28.3	29.3	24.6
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	19.5	19.1	24.8
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	28.3	29.3	24.6
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	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	0.0
U S Stock And Bond Mutual Funds [Member] \| Fair Value, Recurring [Member] \| Spire Missouri [Member]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	19.5	19.1	24.8
U S Stock And Bond Mutual Funds [Member] \| Fair Value, Recurring [Member] \| Spire Missouri [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	19.5	19.1	24.8
U S Stock And Bond Mutual Funds [Member] \| Fair Value, Recurring [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] \| Fair Value, Recurring [Member]
Effects of netting, liability			(8.6)
Derivative Liabilities			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	(0.4)	(57.8)	(2.4)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(35.7)	(30.7)	(8.6)
Derivative Liabilities	0.0	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	(52.0)	(91.8)	(67.0)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(58.4)	(82.3)	(40.2)
Derivative Liabilities	0.0	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]
Derivative Liability Fair Value Gross Liability			8.6
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	0.4	57.8	2.4
Derivative Liability Fair Value Gross Liability	35.7	30.7	8.6
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	52.0	91.8	0.0
Derivative Liability Fair Value Gross Liability	58.4	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]
Derivative Liability Fair Value Gross Liability			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	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, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	0.0	0.0	67.0
Derivative Liability Fair Value Gross Liability	0.0	0.0	40.2
N Y M E X I C E Natural Gas Contracts [Member] \| Fair Value, Recurring [Member] \| Spire Missouri [Member]
Effects of netting, asset	(0.4)	(57.8)	(2.4)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(35.7)	(30.7)
Derivative Liabilities	0.0	0.0
N Y M E X I C E Natural Gas Contracts [Member] \| Fair Value, Recurring [Member] \| Spire Missouri [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	0.4	57.8	2.4
Derivative Liability Fair Value Gross Liability	35.7	30.7
N Y M E X I C E Natural Gas Contracts [Member] \| Fair Value, Recurring [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
Gasoline And Heating Oil Contracts [Member] \| Spire Missouri [Member] \| Estimate of Fair Value Measurement [Member]
Effects of netting, liability	(0.2)
Derivative Liabilities	0.0
Gasoline And Heating Oil Contracts [Member] \| Spire Missouri [Member] \| Fair Value, Inputs, Level 1 [Member] \| Reported Value Measurement [Member]
Derivative Liability Fair Value Gross Liability	0.2
Gasoline And Heating Oil Contracts [Member] \| Spire Missouri [Member] \| Fair Value, Inputs, Level 2 [Member] \| Estimate of Fair Value Measurement [Member]
Derivative Liability Fair Value Gross Liability	0.0
Gasoline And Heating Oil Contracts [Member] \| Fair Value, Recurring [Member] \| Spire Missouri [Member]
Effects of netting, asset	(0.2)
Derivative Assets	0.0
Gasoline And Heating Oil Contracts [Member] \| Fair Value, Recurring [Member] \| Spire Missouri [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset	0.2
Gasoline And Heating Oil Contracts [Member] \| Fair Value, Recurring [Member] \| Spire Missouri [Member] \| Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset	0.0
Natural Gas Commodity Contracts [Member] \| Fair Value, Recurring [Member] \| Gas Marketing [Member]
Effects of netting, asset	(13.2)	(4.0)	(2.3)
Derivative Assets	79.7	52.6	38.1
Effects of netting, liability	(13.2)	(4.0)	(2.3)
Derivative Liabilities	57.9	61.5	75.6
Natural Gas Commodity Contracts [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Gas Marketing [Member]
Derivative Asset Fair Value Gross Asset	92.9	56.6	0.0
Derivative Liability Fair Value Gross Liability	71.1	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	0.0	40.4
Derivative Liability Fair Value Gross Liability	0.0	0.0	77.9
Interest Rate Swap [Member] \| Fair Value, Recurring [Member]
Effects of netting, asset			(5.6)
Derivative Assets			3.8
Interest Rate Swap [Member] \| Fair Value, Recurring [Member] \| Other Segments [Member]
Effects of netting, asset	0.0	0.0
Derivative Assets	61.4	63.6
Effects of netting, liability	0.0		(5.6)
Derivative Liabilities	0.8		1.7
Interest Rate Swap [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Derivative Asset Fair Value Gross Asset			9.4
Interest Rate Swap [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Other Segments [Member]
Derivative Asset Fair Value Gross Asset	61.4	63.6
Derivative Liability Fair Value Gross Liability	0.8		7.3
Interest Rate Swap [Member] \| Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Derivative Asset Fair Value Gross Asset			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	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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- Definition

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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Fair value, after the 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 not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or 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 liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of asset associated with financial liability or 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 right to receive cash collateral under master netting arrangements.

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Fair value, before effects of master netting arrangements, of a financial liability or 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 liabilities not subject to a master netting arrangement and elected not to be offset.

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