DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Gains (Losses) on Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2012
Designated as Hedging Instrument | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	$ 2.5		$ (0.3)		$ 8.4
Ineffective portion of gain (loss) on derivatives recognized in income	(0.2)		(0.8)		(0.1)
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	(4.5)		5.0		4.8
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	2.7		(0.5)		8.4
Ineffective portion of gain (loss) on derivatives recognized in income	0		(0.7)		(0.3)
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	(4.6)		4.9		4.5
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	4.2		0		18.9
Ineffective portion of gain (loss) on derivatives recognized in income	(0.1)		(0.4)		0
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	(1.5)		(0.5)		(10.5)
Ineffective portion of gain (loss) on derivatives recognized in income	0.1		(0.3)		(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	0.1		0.1		0.3
Designated as Hedging Instrument | Gasoline and heating oil contracts | Gas Utility Other Operation Expenses | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	(0.2)		0.2		0
Ineffective portion of gain (loss) on derivatives recognized in income	(0.2)		(0.1)		0.2
Not Designated as Hedging Instrument
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	(5.7)	[1]	(0.8)	[1]	3.2	[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	[1]	0.1	[1]	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	(8.7)	[1]	(0.9)	[1]	3.1	[1]
Not Designated as Hedging Instrument | Natural Gas Commodity Contracts | Gas Marketing Operating Expenses
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	0	[1]	0	[1]	0.1	[1]
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	3.0	[1]	0	[1]	0	[1]
Laclede Gas | Designated as Hedging Instrument | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	(0.2)		0.2		0
Ineffective portion of gain (loss) on derivatives recognized in income	(0.2)		(0.1)		0.2
Laclede Gas | 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	0.1		0.1		0.3
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	0.1		0.1		0.3
Laclede Gas | Designated as Hedging Instrument | Gasoline and heating oil contracts | Gas Utility Other Operation Expenses | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	(0.2)		0.2		0
Ineffective portion of gain (loss) on derivatives recognized in income	(0.2)		(0.1)		0.2
Laclede Gas | Not Designated as Hedging Instrument | Gasoline and heating oil contracts
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	0		0.1		0
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		$ 0.1		$ 0

[1] Gains and losses on Laclede Gas’ natural gas derivative instruments, which are not designated as hedging instruments for financial reporting purposes, are deferred pursuant to the Missouri Utility’s PGA clauses and initially recorded as regulatory assets or regulatory liabilities. These gains and losses are excluded from the table above because they have no direct impact on the Statements of Consolidated Income. Such amounts are recognized in the Statements of Consolidated Income as a component of Regulated Gas Distribution Natural and Propane Gas operating expenses when they are recovered through the PGA clause and reflected in customer billings.