Fair Value Measurements - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
ASSETS
Effects of netting and cash margin receivables/payables	$ (12.8)	$ (32.0)	$ (9.7)
Derivative asset after the effects of netting	68.9	57.7	54.2
LIABILITIES
Effects of netting and cash margin receivables/payables	(14.1)	(23.0)	(28.8)
Derivative liability after the effects of netting	56.2	76.5	41.1
Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	(0.4)	(6.6)
Derivative asset after the effects of netting	23.4	21.9
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	23.4	21.9	21.2
Gasoline and heating oil contracts | Spire Missouri
ASSETS
Effects of netting and cash margin receivables/payables	(0.4)	(0.3)
Derivative asset after the effects of netting	0.0	0.0
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	(4.9)	(0.9)	(16.0)
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	14.2	18.6	16.6
Other | Interest Rate Swap
ASSETS
Effects of netting and cash margin receivables/payables	0.0
Derivative asset after the effects of netting	7.0
LIABILITIES
Effects of netting and cash margin receivables/payables	0.0	0.0	0.0
Derivative liability after the effects of netting	44.0	54.2	24.5
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	23.4	21.9	21.2
Gas Utility | Operating Segments | Gasoline and heating oil contracts
ASSETS
Effects of netting and cash margin receivables/payables	(0.4)	(0.3)
Derivative asset after the effects of netting	0.0	0.0
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	(4.9)	(0.9)	(16.0)
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	(12.4)	(25.4)	(5.3)
Derivative asset after the effects of netting	9.4	2.3	0.2
LIABILITIES
Effects of netting and cash margin receivables/payables	(9.2)	(22.1)	(8.4)
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	0.0	0.0	(4.4)
Derivative asset after the effects of netting	14.9	14.9	16.2
LIABILITIES
Effects of netting and cash margin receivables/payables	0.0	0.0	(4.4)
Derivative liability after the effects of netting	12.2	22.3	16.6
Quoted Prices in Active Markets (Level 1)
ASSETS
Derivative asset before effects of netting	38.3	47.1	38.4
LIABILITIES
Derivative liability before effects of netting	4.9	1.6	16.4
Quoted Prices in Active Markets (Level 1) | Spire Missouri
ASSETS
Derivative asset before effects of netting	23.8	28.5
Quoted Prices in Active Markets (Level 1) | U.S. stock/bond mutual funds | Spire Missouri
ASSETS
Derivative asset before effects of netting	23.4	21.9	21.2
Quoted Prices in Active Markets (Level 1) | Gasoline and heating oil contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.4	0.3
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	4.9	0.9	16.0
Quoted Prices in Active Markets (Level 1) | Other | U.S. stock/bond mutual funds
ASSETS
Derivative asset before effects of netting	14.2	18.6	16.6
Quoted Prices in Active Markets (Level 1) | Other | Interest Rate Swap
ASSETS
Derivative asset before effects of netting	0.0
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	23.4	21.9	21.2
Quoted Prices in Active Markets (Level 1) | Gas Utility | Operating Segments | Gasoline and heating oil contracts
ASSETS
Derivative asset before effects of netting	0.4	0.3
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	4.9	0.9	16.0
Quoted Prices in Active Markets (Level 1) | Gas Marketing | Operating Segments | NYMEX/ICE natural gas contracts
ASSETS
Derivative asset before effects of netting	0.3	0.0	0.6
LIABILITIES
Derivative liability before effects of netting	0.0	0.7	0.4
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	43.4	42.2	25.5
LIABILITIES
Derivative liability before effects of netting	65.4	97.9	52.0
Significant Observable Inputs (Level 2) | Spire Missouri
ASSETS
Derivative asset before effects of netting	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) | Gasoline and heating oil contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting	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 Swap
ASSETS
Derivative asset before effects of netting	7.0
LIABILITIES
Derivative liability before effects of netting	44.0	54.2	24.5
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 | Gasoline and heating oil contracts
ASSETS
Derivative asset before effects of netting	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	21.5	27.7	4.9
LIABILITIES
Derivative liability before effects of netting	9.2	21.4	8.0
Significant Observable Inputs (Level 2) | Gas Marketing | Operating Segments | Natural gas commodity contracts
ASSETS
Derivative asset before effects of netting	14.9	14.5	20.6
LIABILITIES
Derivative liability before effects of netting	12.2	22.3	19.5
Significant Unobservable Inputs (Level 3)
ASSETS
Derivative asset before effects of netting	0.0	0.4	0.0
LIABILITIES
Derivative liability before effects of netting	0.0	0.0	1.5
Significant Unobservable Inputs (Level 3) | Spire Missouri
ASSETS
Derivative asset before effects of netting	0.0	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) | Gasoline and heating oil contracts | Spire Missouri
ASSETS
Derivative asset before effects of netting	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 Swap
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 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 | Gasoline and heating oil contracts
ASSETS
Derivative asset before effects of netting	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	0.4	0.0
LIABILITIES
Derivative liability before effects of netting	$ 0.0	$ 0.0	$ 1.5