DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Derivative Instruments Reconciliation (Details) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	$ 13.5	$ 7.7
Fair value of cash margin receivables offset with derivatives	13.9	3.0
Netting of assets and liabilities with the same counterparty	(22.2)	(7.9)
Derivative instrument assets	4.6	3.2
Deferred Charges – Other	0.6	(0.4)
Total assets (liabilities)	5.2	2.8
Fair value of liability derivatives presented above	29.8	12.0
Current Liabilities – Other	6.8	0.0
Deferred Credits – Other	0.8	4.1
Total liabilities	7.6	4.1
Laclede Gas
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives presented above	1.2	2.5
Fair value of cash margin receivables offset with derivatives	15.5	3.0
Netting of assets and liabilities with the same counterparty	(16.7)	(5.9)
Derivative instrument assets	0.0	(0.4)
Total assets (liabilities)	0.0	(0.4)
Fair value of liability derivatives presented above	22.6	9.5
Current Liabilities – Other	5.7	0.0
Deferred Credits – Other	0.2	3.6
Total liabilities	$ 5.9	$ 3.6