DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Reconciliation to Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
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
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
Spire Missouri
Derivative [Line Items]
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
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