Fair Value Measurements - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021	Mar. 31, 2021
ASSETS
Effects of netting and cash margin receivables/payables	$ (156.2)	$ (208.4)	$ (13.0)
Derivative asset after the effects of netting	105.5	108.1	86.4
LIABILITIES
Effects of netting and cash margin receivables/payables	(49.3)	(73.0)	(6.5)
Derivative liability after the effects of netting	105.2	91.7	45.9
Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	(58.9)	(104.0)	(0.7)
Derivative asset after the effects of netting	23.0	23.8	23.5
U.S. stock/bond mutual funds \| Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0	0.0
Derivative asset after the effects of netting	23.0	23.8	23.5
NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	(58.9)	(104.0)	(0.4)
Derivative asset after the effects of netting	0.0	0.0	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables		(0.3)	(1.8)
Derivative liability after the effects of netting		0.0	0.0
Gasoline And Heating Oil Contracts \| Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables			(0.3)
Derivative asset after the effects of netting			0.0
Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(58.9)
Derivative asset after the effects of netting	0.0
Other \| U.S. stock/bond mutual funds
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0	0.0
Derivative asset after the effects of netting	24.1	26.2	22.9
Other \| Interest Rate Swap
ASSETS
Effects of netting and cash margin receivables/payables	(7.9)	(5.2)	0.0
Derivative asset after the effects of netting	20.9	7.4	27.9
LIABILITIES
Effects of netting and cash margin receivables/payables	(7.9)	(5.2)	0.0
Derivative liability after the effects of netting	0.0	0.5	21.4
Gas Utility \| Operating Segments \| U.S. stock/bond mutual funds
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0	0.0
Derivative asset after the effects of netting	23.0	23.8	23.5
Gas Utility \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables		(104.0)	(0.4)
Derivative asset after the effects of netting		0.0	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables		(0.3)	(1.8)
Derivative liability after the effects of netting		0.0	0.0
Gas Utility \| Operating Segments \| Gasoline And Heating Oil Contracts
ASSETS
Effects of netting and cash margin receivables/payables			(0.3)
Derivative asset after the effects of netting			0.0
Gas Marketing \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(88.4)	(93.7)	(12.3)
Derivative asset after the effects of netting	7.7	21.0	2.8
LIABILITIES
Effects of netting and cash margin receivables/payables	(40.4)	(62.0)	(4.7)
Derivative liability after the effects of netting	0.0	0.0	0.0
Gas Marketing \| Operating Segments \| Natural gas commodity contracts
ASSETS
Effects of netting and cash margin receivables/payables	(1.0)	(5.5)	0.0
Derivative asset after the effects of netting	29.8	29.7	9.3
LIABILITIES
Effects of netting and cash margin receivables/payables	(1.0)	(5.5)	0.0
Derivative liability after the effects of netting	105.2	91.2	24.5
Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	134.8	166.6	47.1
LIABILITIES
Derivative liability before effects of netting	7.9	6.0	1.8
Quoted Prices in Active Markets (Level 1) \| Spire Missouri
ASSETS
Derivative asset before effects of netting	81.9	127.8	24.2
Quoted Prices in Active Markets (Level 1) \| U.S. stock/bond mutual funds \| Spire Missouri
ASSETS
Derivative asset before effects of netting	23.0	23.8	23.5
Quoted Prices in Active Markets (Level 1) \| NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Derivative asset before effects of netting	58.9	104.0	0.4
LIABILITIES
Derivative liability before effects of netting		0.3	1.8
Quoted Prices in Active Markets (Level 1) \| Gasoline And Heating Oil Contracts \| Spire Missouri
ASSETS
Derivative asset before effects of netting			0.3
Quoted Prices in Active Markets (Level 1) \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	58.9
Quoted Prices in Active Markets (Level 1) \| Other \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	24.1	26.2	22.9
Quoted Prices in Active Markets (Level 1) \| Other \| Interest Rate Swap
ASSETS
Derivative asset before effects of netting	28.8	12.6	0.0
LIABILITIES
Derivative liability before effects of netting	7.9	5.7	0.0
Quoted Prices in Active Markets (Level 1) \| Gas Utility \| Operating Segments \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	23.0	23.8	23.5
Quoted Prices in Active Markets (Level 1) \| Gas Utility \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting		104.0	0.4
LIABILITIES
Derivative liability before effects of netting		0.3	1.8
Quoted Prices in Active Markets (Level 1) \| Gas Utility \| Operating Segments \| Gasoline And Heating Oil Contracts
ASSETS
Derivative asset before effects of netting			0.3
Quoted Prices in Active Markets (Level 1) \| Gas Marketing \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Quoted Prices in Active Markets (Level 1) \| Gas Marketing \| Operating Segments \| Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	126.9	149.9	52.3
LIABILITIES
Derivative liability before effects of netting	146.6	158.7	50.6
Significant Observable Inputs (Level 2) \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
Significant Observable Inputs (Level 2) \| U.S. stock/bond mutual funds \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
Significant Observable Inputs (Level 2) \| NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting		0.0	0.0
Significant Observable Inputs (Level 2) \| Gasoline And Heating Oil Contracts \| Spire Missouri
ASSETS
Derivative asset before effects of netting			0.0
Significant Observable Inputs (Level 2) \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	0.0
Significant Observable Inputs (Level 2) \| Other \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
Significant Observable Inputs (Level 2) \| Other \| Interest Rate Swap
ASSETS
Derivative asset before effects of netting	0.0	0.0	27.9
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	21.4
Significant Observable Inputs (Level 2) \| Gas Utility \| Operating Segments \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
Significant Observable Inputs (Level 2) \| Gas Utility \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting		0.0	0.0
LIABILITIES
Derivative liability before effects of netting		0.0	0.0
Significant Observable Inputs (Level 2) \| Gas Utility \| Operating Segments \| Gasoline And Heating Oil Contracts
ASSETS
Derivative asset before effects of netting			0.0
Significant Observable Inputs (Level 2) \| Gas Marketing \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	96.1	114.7	15.1
LIABILITIES
Derivative liability before effects of netting	40.4	62.0	4.7
Significant Observable Inputs (Level 2) \| Gas Marketing \| Operating Segments \| Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	30.8	35.2	9.3
LIABILITIES
Derivative liability before effects of netting	$ 106.2	$ 96.7	$ 24.5

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