Derivative Instruments and Hedging Activities - Reconciliation to Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Derivative [Line Items]
Fair value of derivative assets presented above	$ 24.2	$ 27.4
Netting of assets and liabilities with the same counterparty	(23.7)	(14.8)
Derivative asset after the effects of netting	14.8	19.0
Fair value of derivative liabilities presented above	78.5	21.0
Netting of assets and liabilities with the same counterparty	(23.7)	(14.8)
Derivative liability after the effects of netting	54.8	6.2
Spire Missouri
Derivative [Line Items]
Fair value of derivative assets presented above	0.0	2.7
Fair value of cash margin (payable) receivable offset with derivatives	12.3	(0.8)
Netting of assets and liabilities with the same counterparty	(12.3)	(1.9)
Derivative asset after the effects of netting	0.0	0.0
Fair value of derivative liabilities presented above	12.3	1.9
Netting of assets and liabilities with the same counterparty	(12.3)	(1.9)
Derivative liability after the effects of netting	$ 0.0	$ 0.0