Fair Value Measurements - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
ASSETS
Derivative asset after the effects of netting	$ 17.2	$ 14.8
LIABILITIES
Derivative liability after the effects of netting	76.5	54.8
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	(32.0)	(9.4)
Derivative asset after the effects of netting	57.7	50.8
LIABILITIES
Effects of netting and cash margin receivables/payables	(23.0)	(23.7)
Derivative liability after the effects of netting	76.5	54.8
Fair Value, Measurements, Recurring | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	(6.6)
Derivative asset after the effects of netting	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	21.9	20.5
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	(6.3)
Derivative asset after the effects of netting	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(0.9)	(12.3)
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	18.6	15.5
Fair Value, Measurements, Recurring | Other | Interest Rate Swap
LIABILITIES
Effects of netting and cash margin receivables/payables	0.0	0.0
Derivative liability after the effects of netting	54.2	43.4
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	21.9	20.5
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	(6.3)
Derivative asset after the effects of netting	0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(0.9)	(12.3)
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	(25.4)	(6.9)
Derivative asset after the effects of netting	2.3	0.5
LIABILITIES
Effects of netting and cash margin receivables/payables	(22.1)	(8.9)
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	0.0	(2.5)
Derivative asset after the effects of netting	14.9	14.3
LIABILITIES
Effects of netting and cash margin receivables/payables	0.0	(2.5)
Derivative liability after the effects of netting	22.3	11.4
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	47.1	36.9
LIABILITIES
Derivative liability before effects of netting	1.6	12.7
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before effects of netting	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	21.9	20.5
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	6.3
LIABILITIES
Derivative liability before effects of netting	0.9	12.3
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	18.6	15.5
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Other | Interest Rate Swap
LIABILITIES
Derivative liability before effects of netting	0.0	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	21.9	20.5
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	6.3
LIABILITIES
Derivative liability before effects of netting	0.9	12.3
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	0.9
LIABILITIES
Derivative liability before effects of netting	0.7	0.4
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	42.2	23.3
LIABILITIES
Derivative liability before effects of netting	97.9	65.7
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before effects of netting	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
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
LIABILITIES
Derivative liability before effects of netting	54.2	43.4
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
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	27.7	6.5
LIABILITIES
Derivative liability before effects of netting	21.4	8.5
Fair Value, Measurements, Recurring | Significant Observable Inputs (Level 2) | Gas Marketing | Operating Segments | Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	14.5	16.8
LIABILITIES
Derivative liability before effects of netting	22.3	13.8
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.4	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.1
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Spire Missouri
ASSETS
Derivative asset before effects of netting	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
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
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
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	0.4	0.0
LIABILITIES
Derivative liability before effects of netting	$ 0.0	$ 0.1