Derivative Instruments and Hedging Activities - Fair Value of Derivative Instruments in the Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Derivative [Line Items]
Fair value of derivative assets presented above	$ 49.2	$ 24.2
Fair value of derivative liabilities presented above	99.5	78.5
Not Designated as Hedging Instrument
Derivative [Line Items]
Fair value of derivative assets presented above	49.2	24.2
Fair value of derivative liabilities presented above	45.3	35.1
Not Designated as Hedging Instrument | Derivative Instruments Assets | Gasoline and Heating Oil Contracts
Derivative [Line Items]
Fair value of derivative assets presented above	0.3
Fair value of derivative liabilities presented above	0.0
Spire Missouri
Derivative [Line Items]
Fair value of derivative assets presented above	6.6	0.0
Fair value of derivative liabilities presented above	0.9	12.3
Spire Missouri | Not Designated as Hedging Instrument
Derivative [Line Items]
Fair value of derivative assets presented above	6.6
Fair value of derivative liabilities presented above	0.9
Spire Missouri | Not Designated as Hedging Instrument | Accounts Receivable – Other | Gas Utility | Natural Gas Contracts
Derivative [Line Items]
Fair value of derivative assets presented above	6.3	0.0
Fair value of derivative liabilities presented above	0.9	$ 12.3
Spire Missouri | Not Designated as Hedging Instrument | Derivative Instruments Assets | Gas Utility | Gasoline and Heating Oil Contracts
Derivative [Line Items]
Fair value of derivative assets presented above	0.3
Fair value of derivative liabilities presented above	$ 0.0