DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Effect of Derivative Instruments on the Statement of Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Derivative [Line Items]
Net hedging gain (loss) arising during the period	$ 3.9	$ 11.5	$ (4.0)
Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships
Derivative [Line Items]
Effective portion of gain (loss) reclassified from AOCI to income	1.5	0.0	(1.1)
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.5	0.3
Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Other Comprehensive Income (Loss)
Derivative [Line Items]
Net hedging gain (loss) arising during the period	3.9	11.5	(4.0)
Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Other Comprehensive Income (Loss) | Gasoline and heating oil contracts
Derivative [Line Items]
Net hedging gain (loss) arising during the period	0.0	0.1	0.0
Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Gas Utility Other Operating Expenses | Gasoline and heating oil contracts
Derivative [Line Items]
Effective portion of gain (loss) reclassified from AOCI to income	0.1	0.2	(0.5)
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.0	0.1
Spire Missouri
Derivative [Line Items]
Net hedging gain (loss) arising during the period	0.0	0.1	0.0
Spire Missouri | Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Other Comprehensive Income (Loss) | Gasoline and heating oil contracts
Derivative [Line Items]
Net hedging gain (loss) arising during the period	0.0	0.1	0.0
Spire Missouri | Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Gas Utility Other Operating Expenses | Gasoline and heating oil contracts
Derivative [Line Items]
Effective portion of gain (loss) reclassified from AOCI to income	0.1	0.2	(0.5)
Ineffective portion of gain (loss) on derivatives recognized in income	$ 0.0	$ 0.0	$ 0.1