DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Gains (Losses) on Derivative Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Designated as Hedging Instrument | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	$ 0.0	$ (1.1)	$ (4.4)
Ineffective portion of gain (loss) on derivatives recognized in income	0.5	0.3	(0.4)
Designated as Hedging Instrument | Cash Flow Hedging | Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) recognized in OCI on derivatives	11.5	(4.0)	(5.5)
Designated as Hedging Instrument | Cash Flow Hedging | Natural gas contracts
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	(0.3)	(0.6)	(3.5)
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.2	(0.5)
Designated as Hedging Instrument | Cash Flow Hedging | Natural gas contracts | Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) recognized in OCI on derivatives	0.0	(0.6)	(4.3)
Designated as Hedging Instrument | Cash Flow Hedging | Natural gas contracts | Gas Marketing Operating Revenues
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	(0.4)	4.3	1.7
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.1	0.0
Designated as Hedging Instrument | Cash Flow Hedging | Natural gas contracts | Gas Marketing Operating Expenses
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	0.1	(4.9)	(5.2)
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.1	(0.5)
Designated as Hedging Instrument | Cash Flow Hedging | Gasoline and heating oil contracts | Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) recognized in OCI on derivatives	0.1	0.0	(1.2)
Designated as Hedging Instrument | Cash Flow Hedging | Gasoline and heating oil contracts | Gas Utility Other Operating Expenses
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	0.2	(0.5)	(0.9)
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.1	0.1
Designated as Hedging Instrument | Cash Flow Hedging | Interest rate swaps | Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) recognized in OCI on derivatives	11.4	(3.4)	0.0
Designated as Hedging Instrument | Cash Flow Hedging | Interest rate swaps | Interest Expense
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	0.1	0.0	0.0
Ineffective portion of gain (loss) on derivatives recognized in income	0.5	0.0	0.0
Not Designated as Hedging Instrument
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	(3.7)	10.6	(11.1)
Not Designated as Hedging Instrument | Gasoline and heating oil contracts | Other Income and (Income Deductions) - Net
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	0.0	0.0	(0.2)
Not Designated as Hedging Instrument | Natural gas commodity contracts | Gas Marketing Operating Revenues
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	0.7	12.3	(1.3)
Not Designated as Hedging Instrument | NYMEX / ICE natural gas contracts | Gas Marketing Operating Revenues
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	(4.4)	(1.7)	(9.6)
Spire Missouri | Designated as Hedging Instrument | Cash Flow Hedging | Gasoline and heating oil contracts | Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) recognized in OCI on derivatives	0.1	0.0	(1.2)
Spire Missouri | Designated as Hedging Instrument | Cash Flow Hedging | Gasoline and heating oil contracts | Gas Utility Other Operating Expenses
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	0.2	(0.5)	(0.9)
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.1	0.1
Spire Missouri | Not Designated as Hedging Instrument | Gasoline and heating oil contracts | Other Income and (Income Deductions) - Net
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	$ 0.0	$ 0.0	$ (0.2)