Derivative Instruments and Hedging Activities - Reconciliation to Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Derivative [Line Items]
Fair value of derivative assets presented above	$ 49.2	$ 24.2
Netting of assets and liabilities with the same counterparty	(23.0)	(23.7)
Derivative asset after the effects of netting	17.2	14.8
Fair value of derivative liabilities presented above	99.5	78.5
Netting of assets and liabilities with the same counterparty	(23.0)	(23.7)
Derivative liability after the effects of netting	76.5	54.8
Spire Missouri
Derivative [Line Items]
Fair value of derivative assets presented above	6.6	0.0
Fair value of cash margin (payable) receivable offset with derivatives	(5.7)	12.3
Netting of assets and liabilities with the same counterparty	(0.9)	(12.3)
Derivative asset after the effects of netting	0.0	0.0
Fair value of derivative liabilities presented above	0.9	12.3
Netting of assets and liabilities with the same counterparty	(0.9)	(12.3)
Derivative liability after the effects of netting	$ 0.0	$ 0.0