DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Derivative Instruments Reconciliation (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Offsetting Derivative Assets [Abstract]
Fair value of asset derivatives presented above	$ 27.4	$ 13.0
Fair value of cash margin (payable) receivable offset with derivatives	6.4	(1.5)
Netting of assets and liabilities with the same counterparty	(14.8)	(5.3)
Derivative asset after the effects of netting	19.0	6.2
Derivative instrument assets	13.3	5.9
Deferred Charges – Other	5.7	0.3
Offsetting Derivative Liabilities [Abstract]
Fair value of liability derivatives presented above	21.0	13.3
Netting of assets and liabilities with the same counterparty	(14.8)	(5.3)
Derivative liability after the effects of netting	6.2	8.0
Current Liabilities – Other	6.0	4.9
Deferred Credits – Other	0.2	3.1
Spire Missouri
Offsetting Derivative Assets [Abstract]
Fair value of asset derivatives presented above	2.7	3.5
Fair value of cash margin (payable) receivable offset with derivatives	(0.8)	(1.5)
Netting of assets and liabilities with the same counterparty	(1.9)	(1.9)
Derivative asset after the effects of netting	0.0	0.1
Derivative instrument assets	0.0	0.1
Offsetting Derivative Liabilities [Abstract]
Fair value of liability derivatives presented above	1.9	1.9
Netting of assets and liabilities with the same counterparty	(1.9)	(1.9)
Derivative liability after the effects of netting	$ 0.0	$ 0.0