DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Fair Value of Derivative Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	$ 27.4	$ 13.0
Fair value of liability derivatives presented above	21.0	13.3
Derivatives in Cash Flow Hedging Relationships
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	3.0
Fair value of liability derivatives presented above	0.0
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	24.4	12.3
Fair value of liability derivatives presented above	21.0	12.2
Other | Interest rate swaps | Derivatives in Cash Flow Hedging Relationships | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	3.0	0.0
Fair value of liability derivatives presented above	0.0	0.9
Gas Utility | Operating Segments | Gasoline and heating oil contracts | Derivatives in Cash Flow Hedging Relationships | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.1
Fair value of liability derivatives presented above		0.0
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	2.7	3.4
Fair value of liability derivatives presented above	1.9	1.9
Gas Marketing | Derivatives in Cash Flow Hedging Relationships
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.7
Fair value of liability derivatives presented above		1.1
Gas Marketing | Operating Segments | Natural gas contracts | Derivatives in Cash Flow Hedging Relationships | Derivative Instrument Assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.3
Fair value of liability derivatives presented above		0.2
Gas Marketing | Operating Segments | Natural gas contracts | Derivatives in Cash Flow Hedging Relationships | Deferred Charges – Other
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above		0.3
Fair value of liability derivatives presented above		0.0
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	3.8	1.7
Fair value of liability derivatives presented above	10.7	1.4
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	0.4	0.3
Fair value of liability derivatives presented above	0.7	0.5
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	10.9	5.3
Fair value of liability derivatives presented above	1.0	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	6.3	0.4
Fair value of liability derivatives presented above	0.2	0.0
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	0.3	0.8
Fair value of liability derivatives presented above	6.3	5.0
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	0.0	0.4
Fair value of liability derivatives presented above	$ 0.2	$ 3.3