DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Fair Value of Derivative Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	$ 18.4	$ 13.5
Fair value of liability derivatives presented above	12.5	29.8
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	3.2
Fair value of liability derivatives presented above	3.9
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	15.2	8.3
Fair value of liability derivatives presented above	8.6	25.8
Other | Interest rate swaps | Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.0
Gas Utility | Operating Segments | Gasoline and heating oil contracts | Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.3
Fair value of liability derivatives presented above	0.0
Gas Utility | Operating Segments | Gasoline and heating oil contracts | Designated as Hedging Instrument | Accounts Receivable – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.0
Fair value of liability derivatives presented above		0.3
Gas Utility | Operating Segments | Natural gas contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.0	0.0
Fair value of liability derivatives presented above	0.2	5.7
Gas Utility | Operating Segments | Natural gas contracts | Not Designated as Hedging Instrument | Accounts Receivable – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	5.4	1.2
Fair value of liability derivatives presented above	1.6	16.4
Gas Utility | Operating Segments | Natural gas contracts | Not Designated as Hedging Instrument | Deferred Charges – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.0
Fair value of liability derivatives presented above		0.2
Gas Marketing | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		5.2
Fair value of liability derivatives presented above		4.0
Gas Marketing | Operating Segments | Natural gas contracts | Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	2.5	4.1
Fair value of liability derivatives presented above	0.8	3.2
Gas Marketing | Operating Segments | Natural gas contracts | Designated as Hedging Instrument | Deferred Charges – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.4	1.1
Fair value of liability derivatives presented above	0.1	0.5
Gas Marketing | Operating Segments | NYMEX / ICE natural gas contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.8	4.7
Fair value of liability derivatives presented above	4.1	0.6
Gas Marketing | Operating Segments | NYMEX / ICE natural gas contracts | Not Designated as Hedging Instrument | Deferred Charges – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.0	0.7
Fair value of liability derivatives presented above	0.1	0.7
Gas Marketing | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	6.5	1.4
Fair value of liability derivatives presented above	0.2	0.1
Gas Marketing | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Deferred Charges – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	2.1
Fair value of liability derivatives presented above	0.3
Gas Marketing | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Current Liabilities – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.2	0.2
Fair value of liability derivatives presented above	2.0	1.4
Gas Marketing | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Deferred Credits – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.2	0.1
Fair value of liability derivatives presented above	0.1	0.7
Other Segments | Other | Interest rate swaps | Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of liability derivatives presented above	3.0
Laclede Gas
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	5.7	1.2
Fair value of liability derivatives presented above	1.8	22.6
Laclede Gas | Gas Utility
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	5.7	1.2
Fair value of liability derivatives presented above	1.8	22.6
Laclede Gas | Gas Utility | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.3	0.0
Fair value of liability derivatives presented above	0.0	0.3
Laclede Gas | Gas Utility | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	5.4	1.2
Fair value of liability derivatives presented above	1.8	22.3
Laclede Gas | Gas Utility | Gasoline and heating oil contracts | Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.3
Fair value of liability derivatives presented above	0.0
Laclede Gas | Gas Utility | Gasoline and heating oil contracts | Designated as Hedging Instrument | Accounts Receivable – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.0
Fair value of liability derivatives presented above		0.3
Laclede Gas | Gas Utility | Natural gas contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.0
Fair value of liability derivatives presented above		5.7
Laclede Gas | Gas Utility | Natural gas contracts | Not Designated as Hedging Instrument | Accounts Receivable – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	5.4	1.2
Fair value of liability derivatives presented above	1.6	16.4
Laclede Gas | Gas Utility | Natural gas contracts | Not Designated as Hedging Instrument | Deferred Charges – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.0
Fair value of liability derivatives presented above		$ 0.2
Laclede Gas | Gas Utility | OTCBB natural gas contracts | Not Designated as Hedging Instrument | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	0.0
Fair value of liability derivatives presented above	$ 0.2