Fair Value Measurements - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
ASSETS
Effects of netting and cash margin receivables/payables	$ (9.7)	$ (9.4)	$ (18.8)
Derivative asset after the effects of netting	54.2	50.8	56.1
LIABILITIES
Effects of netting and cash margin receivables/payables	(28.8)	(23.7)	(18.6)
Derivative liability after the effects of netting	41.1	54.8	24.6
Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables			(1.2)
Derivative asset after the effects of netting			18.3
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	21.2	20.5	18.3
NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables			(1.2)
Derivative asset after the effects of netting			0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(16.0)	(12.3)	(0.9)
Derivative liability after the effects of netting	0.0	0.0	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	16.6	15.5	12.5
Other \| Interest rate swaps
LIABILITIES
Effects of netting and cash margin receivables/payables	0.0	0.0	0.0
Derivative liability after the effects of netting	24.5	43.4	7.5
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	21.2	20.5	18.3
Gas Utility \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables			(1.2)
Derivative asset after the effects of netting			0.0
LIABILITIES
Effects of netting and cash margin receivables/payables	(16.0)	(12.3)	(0.9)
Derivative liability after the effects of netting	0.0	0.0	0.0
Gas Marketing \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Effects of netting and cash margin receivables/payables	(5.3)	(6.9)	(9.3)
Derivative asset after the effects of netting	0.2	0.5	2.8
LIABILITIES
Effects of netting and cash margin receivables/payables	(8.4)	(8.9)	(9.4)
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	(4.4)	(2.5)	(8.3)
Derivative asset after the effects of netting	16.2	14.3	22.5
LIABILITIES
Effects of netting and cash margin receivables/payables	(4.4)	(2.5)	(8.3)
Derivative liability after the effects of netting	16.6	11.4	17.1
Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	38.4	36.9	32.2
LIABILITIES
Derivative liability before effects of netting	16.4	12.7	1.5
Quoted Prices in Active Markets (Level 1) \| Spire Missouri
ASSETS
Derivative asset before effects of netting			19.5
Quoted Prices in Active Markets (Level 1) \| U.S. stock/bond mutual funds \| Spire Missouri
ASSETS
Derivative asset before effects of netting	21.2	20.5	18.3
Quoted Prices in Active Markets (Level 1) \| NYMEX/ICE natural gas contracts \| Spire Missouri
ASSETS
Derivative asset before effects of netting			1.2
LIABILITIES
Derivative liability before effects of netting	16.0	12.3	0.9
Quoted Prices in Active Markets (Level 1) \| Other \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	16.6	15.5	12.5
Quoted Prices in Active Markets (Level 1) \| Other \| Interest rate swaps
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	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	21.2	20.5	18.3
Quoted Prices in Active Markets (Level 1) \| Gas Utility \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting			1.2
LIABILITIES
Derivative liability before effects of netting	16.0	12.3	0.9
Quoted Prices in Active Markets (Level 1) \| Gas Marketing \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	0.6	0.9	0.2
LIABILITIES
Derivative liability before effects of netting	0.4	0.4	0.6
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	25.5	23.3	42.7
LIABILITIES
Derivative liability before effects of netting	52.0	65.7	41.4
Significant Observable Inputs (Level 2) \| Spire Missouri
ASSETS
Derivative asset before effects of netting			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
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	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 swaps
LIABILITIES
Derivative liability before effects of netting	24.5	43.4	7.5
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
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Significant Observable Inputs (Level 2) \| Gas Marketing \| Operating Segments \| NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	4.9	6.5	11.9
LIABILITIES
Derivative liability before effects of netting	8.0	8.5	8.8
Significant Observable Inputs (Level 2) \| Gas Marketing \| Operating Segments \| Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	20.6	16.8	30.8
LIABILITIES
Derivative liability before effects of netting	19.5	13.8	25.1
Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
LIABILITIES
Derivative liability before effects of netting	1.5	0.1	0.3
Significant Unobservable Inputs (Level 3) \| Spire Missouri
ASSETS
Derivative asset before effects of netting			0.0
Significant Unobservable Inputs (Level 3) \| U.S. stock/bond mutual funds \| Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
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	0.0
Significant Unobservable Inputs (Level 3) \| Other \| U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	0.0	0.0	0.0
Significant Unobservable Inputs (Level 3) \| Other \| Interest rate swaps
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
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	0.0
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	0.0
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	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	0.0
Significant Unobservable Inputs (Level 3) \| 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	$ 1.5	$ 0.1	$ 0.3

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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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