DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Fair Value of Derivative Instruments in the Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
Fair value of asset derivatives presented above	$ 27.4	$ 13.0
Fair value of liability derivatives presented above	21.0	13.3
Not Designated as Hedging Instrument
Derivative [Line Items]
Fair value of asset derivatives presented above	24.4	12.3
Fair value of liability derivatives presented above	21.0	12.2
Derivatives in Cash Flow Hedging Relationships
Derivative [Line Items]
Fair value of asset derivatives presented above	3.0
Fair value of liability derivatives presented above	0.0
Spire Missouri
Derivative [Line Items]
Fair value of asset derivatives presented above	2.7	3.5
Fair value of liability derivatives presented above	1.9	1.9
Spire Missouri | Gas Utility
Derivative [Line Items]
Fair value of asset derivatives presented above	2.7	3.5
Fair value of liability derivatives presented above	1.9	1.9
Spire Missouri | Not Designated as Hedging Instrument | Accounts Receivable – Other | Gas Utility | Natural gas contracts
Derivative [Line Items]
Fair value of asset derivatives presented above	2.7	3.4
Fair value of liability derivatives presented above	$ 1.9	1.9
Spire Missouri | Derivatives in Cash Flow Hedging Relationships | Gas Utility
Derivative [Line Items]
Fair value of asset derivatives presented above		0.1
Fair value of liability derivatives presented above		0.0
Spire Missouri | Derivatives in Cash Flow Hedging Relationships | Derivative Instrument Assets | Gas Utility | Gasoline and heating oil contracts
Derivative [Line Items]
Fair value of asset derivatives presented above		0.1
Fair value of liability derivatives presented above		$ 0.0