DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Gains (Losses) on Derivative Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Designated as Hedging Instrument | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	$ (4.4)	$ 2.5	$ (0.3)
Ineffective portion of gain (loss) on derivatives recognized in income	(0.4)	(0.2)	(0.8)
Designated as Hedging Instrument | Other Comprehensive Income (Loss) | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) recognized in OCI on derivatives	(5.5)	(4.5)	5.0
Designated as Hedging Instrument | Natural gas contracts | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	(3.5)	2.7	(0.5)
Ineffective portion of gain (loss) on derivatives recognized in income	(0.5)	0.0	(0.7)
Designated as Hedging Instrument | Natural gas contracts | Other Comprehensive Income (Loss) | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) recognized in OCI on derivatives	(4.3)	(4.6)	4.9
Designated as Hedging Instrument | Natural gas contracts | Gas Marketing Operating Revenues | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	1.7	4.2	0.0
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	(0.1)	(0.4)
Designated as Hedging Instrument | Natural gas contracts | Gas Marketing Operating Expenses | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	(5.2)	(1.5)	(0.5)
Ineffective portion of gain (loss) on derivatives recognized in income	(0.5)	0.1	(0.3)
Designated as Hedging Instrument | Gasoline and heating oil contracts | Other Comprehensive Income (Loss) | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) recognized in OCI on derivatives	(1.2)	0.1	0.1
Designated as Hedging Instrument | Gasoline and heating oil contracts | Gas Utility Other Operating Expenses | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	(0.9)	(0.2)	0.2
Ineffective portion of gain (loss) on derivatives recognized in income	0.1	(0.2)	(0.1)
Not Designated as Hedging Instrument
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	(11.1)	(5.7)	(0.8)
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	(0.2)	0.0	0.1
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	(1.3)	(8.7)	(0.9)
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	(9.6)	3.0	0.0
Laclede Gas | Designated as Hedging Instrument | Gasoline and heating oil contracts | Other Comprehensive Income (Loss) | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) recognized in OCI on derivatives	(1.2)	0.1	0.1
Laclede Gas | Designated as Hedging Instrument | Gasoline and heating oil contracts | Gas Utility Other Operating Expenses | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	(0.9)	(0.2)	0.2
Ineffective portion of gain (loss) on derivatives recognized in income	0.1	(0.2)	(0.1)
Laclede Gas | 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	$ (0.2)	$ 0.0	$ 0.1