Derivative Instruments and Hedging Activities - Fair Value of Derivative Instruments in the Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Derivative [Line Items]
Fair value of derivative assets presented above	$ 266.5	$ 49.2
Fair value of derivative liabilities presented above	164.7	99.5
Not Designated as Hedging Instrument
Derivative [Line Items]
Fair value of derivative assets presented above	253.9	49.2
Fair value of derivative liabilities presented above	159.0	45.3
Not Designated as Hedging Instrument | Derivative Instrument 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	104.0	6.6
Fair value of derivative liabilities presented above	0.3	0.9
Spire Missouri | Not Designated as Hedging Instrument
Derivative [Line Items]
Fair value of derivative assets presented above	104.0	6.6
Fair value of derivative liabilities presented above	0.3	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	104.0	6.3
Fair value of derivative liabilities presented above	$ 0.3	0.9
Spire Missouri | Not Designated as Hedging Instrument | Derivative Instrument 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