Fair Value Measurements - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
ASSETS
Derivative asset after the effects of netting	$ 58.1	$ 17.2
LIABILITIES
Derivative liability after the effects of netting	91.7	76.5
Spire Missouri
ASSETS
Derivative asset after the effects of netting	0.0	0.0
LIABILITIES
Derivative liability after the effects of netting	0.0	0.0
Fair Value, Measurements, Recurring
ASSETS
Effects of netting and cash margin receivables/payables	(208.4)	(32.0)
Derivative asset after the effects of netting	108.1	57.7
LIABILITIES
Effects of netting and cash margin receivables/payables	(73.0)	(23.0)
Derivative liability after the effects of netting	91.7	76.5
Fair Value, Measurements, Recurring | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	(104.0)	(6.6)
Derivative asset after the effects of netting	23.8	21.9
Fair Value, Measurements, Recurring | U.S. stock/bond mutual funds | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0
Derivative asset after the effects of netting	23.8	21.9
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | NYMEX/ICE natural gas contracts | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	(104.0)	(6.3)
Derivative asset after the effects of netting	0.0	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(0.3)	(0.9)
Derivative liability after the effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Other | U.S. stock/bond mutual funds
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0
Derivative asset after the effects of netting	26.2	18.6
Fair Value, Measurements, Recurring | Other | Interest Rate Swap
ASSETS
Effects of netting and cash margin receivables/payables	(5.2)
Derivative asset after the effects of netting	7.4
LIABILITIES
Effects of netting and cash margin receivables/payables	(5.2)	0.0
Derivative liability after the effects of netting	0.5	54.2
Fair Value, Measurements, Recurring | Gas Utility | Operating Segments | U.S. stock/bond mutual funds
ASSETS
Effects of netting and cash margin receivables/payables	0.0	0.0
Derivative asset after the effects of netting	23.8	21.9
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | Gas Utility | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(104.0)	(6.3)
Derivative asset after the effects of netting	0.0	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(0.3)	(0.9)
Derivative liability after the effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Marketing | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(93.7)	(25.4)
Derivative asset after the effects of netting	21.0	2.3
LIABILITIES
Effects of netting and cash margin receivables/payables	(62.0)	(22.1)
Derivative liability after the effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Gas Marketing | Operating Segments | Natural gas commodity contracts
ASSETS
Effects of netting and cash margin receivables/payables	(5.5)	0.0
Derivative asset after the effects of netting	29.7	14.9
LIABILITIES
Effects of netting and cash margin receivables/payables	(5.5)	0.0
Derivative liability after the effects of netting	91.2	22.3
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	166.6	47.1
LIABILITIES
Derivative liability before effects of netting	6.0	1.6
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before effects of netting	127.8	28.5
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | U.S. stock/bond mutual funds | Spire Missouri
ASSETS
Derivative asset before effects of netting	23.8	21.9
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Gasoline and heating oil contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting		0.3
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | NYMEX/ICE natural gas contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting	104.0	6.3
LIABILITIES
Derivative liability before effects of netting	0.3	0.9
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Other | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	26.2	18.6
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Other | Interest Rate Swap
ASSETS
Derivative asset before effects of netting	12.6
LIABILITIES
Derivative liability before effects of netting	5.7	0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Gas Utility | Operating Segments | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	23.8	21.9
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | 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	6.3
LIABILITIES
Derivative liability before effects of netting	0.3	0.9
Fair Value, Measurements, Recurring | 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
LIABILITIES
Derivative liability before effects of netting	0.0	0.7
Fair Value, Measurements, Recurring | 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
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2)
ASSETS
Derivative asset before effects of netting	149.9	42.2
LIABILITIES
Derivative liability before effects of netting	158.7	97.9
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | U.S. stock/bond mutual funds | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Gasoline and heating oil contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting		0.0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | NYMEX/ICE natural gas contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Other | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Other | Interest Rate Swap
ASSETS
Derivative asset before effects of netting	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	54.2
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Gas Utility | Operating Segments | Gasoline and heating oil contracts
ASSETS
Derivative asset before effects of netting		0.0
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Gas Marketing | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	114.7	27.7
LIABILITIES
Derivative liability before effects of netting	62.0	21.4
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Gas Marketing | Operating Segments | Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	35.2	14.5
LIABILITIES
Derivative liability before effects of netting	96.7	22.3
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.0	0.4
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. stock/bond mutual funds | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Gasoline and heating oil contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting		0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | NYMEX/ICE natural gas contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other | Interest Rate Swap
ASSETS
Derivative asset before effects of netting	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Gas Utility | Operating Segments | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Gas Utility | Operating Segments | Gasoline and heating oil contracts
ASSETS
Derivative asset before effects of netting		0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | 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
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Gas Marketing | 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
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Gas Marketing | Operating Segments | Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	0.0	0.4
LIABILITIES
Derivative liability before effects of netting	$ 0.0	$ 0.0