DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Gains (Losses) on Derivative Instruments (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Derivatives in Cash Flow Hedging Relationships [Abstract]
Net hedging gain (loss) arising during the period	$ 3.9	$ 11.5	$ (4.0)
Derivatives in Cash Flow Hedging Relationships \| Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	1.5	0.0	(1.1)
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.5	0.3
Derivatives in Cash Flow Hedging Relationships \| Cash Flow Hedging \| Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Net hedging gain (loss) arising during the period	3.9	11.5	(4.0)
Derivatives in Cash Flow Hedging Relationships \| Cash Flow Hedging \| Natural gas contracts
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	0.0	(0.3)	(0.6)
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.0	0.2
Derivatives in Cash Flow Hedging Relationships \| Cash Flow Hedging \| Natural gas contracts \| Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Net hedging gain (loss) arising during the period	0.0	0.0	(0.6)
Derivatives in Cash Flow Hedging Relationships \| 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	0.0	(0.4)	4.3
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.0	0.1
Derivatives in Cash Flow Hedging Relationships \| 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	0.0	0.1	(4.9)
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.0	0.1
Derivatives in Cash Flow Hedging Relationships \| Cash Flow Hedging \| Gasoline and heating oil contracts \| Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Net hedging gain (loss) arising during the period	0.0	0.1	0.0
Derivatives in Cash Flow Hedging Relationships \| 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	0.1	0.2	(0.5)
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.0	0.1
Derivatives in Cash Flow Hedging Relationships \| Cash Flow Hedging \| Interest rate swaps \| Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Net hedging gain (loss) arising during the period	3.9	11.4	(3.4)
Derivatives in Cash Flow Hedging Relationships \| 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	1.4	0.1	0.0
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.5	0.0
Not Designated as Hedging Instrument
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	2.1	(3.7)	10.6
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	10.2	0.7	12.3
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	(8.1)	0.0	0.0
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	$ 0.0	$ (4.4)	$ (1.7)

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

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Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

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Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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