DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Derivative Instruments Reconciliation (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Offsetting Derivative Assets [Abstract]
Fair value of asset derivatives presented above	$ 13.0	$ 18.4
Fair value of cash margin receivables offset with derivatives	(1.5)	2.5
Netting of assets and liabilities with the same counterparty	(5.3)	(7.6)
Derivative asset after the effects of netting	6.2	13.3
Derivative instrument assets	5.9	11.4
Deferred Charges – Other	0.3	1.9
Offsetting Derivative Liabilities [Abstract]
Fair value of liability derivatives presented above	13.3	12.5
Netting of assets and liabilities with the same counterparty	(5.3)	(7.6)
Derivative liability after the effects of netting	8.0	4.9
Current Liabilities – Other	4.9	4.8
Deferred Credits – Other	3.1	0.1
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Offsetting Derivative Assets [Abstract]
Fair value of asset derivatives presented above	3.5	5.7
Fair value of cash margin receivables offset with derivatives	(1.5)	0.8
Netting of assets and liabilities with the same counterparty	(1.9)	(1.6)
Derivative asset after the effects of netting	0.1	4.9
Derivative instrument assets	0.1	4.9
Offsetting Derivative Liabilities [Abstract]
Fair value of liability derivatives presented above	1.9	1.8
Netting of assets and liabilities with the same counterparty	(1.9)	(1.6)
Derivative liability after the effects of netting	0.0	0.2
Current Liabilities – Other	$ 0.0	$ 0.2