Derivative Instruments and Hedging Activities - Reconciliation to Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Derivative [Line Items]
Fair value of derivative assets presented above	$ 266.5	$ 49.2
Netting of assets and liabilities with the same counterparty	(73.0)	(23.0)
Derivative asset after the effects of netting	58.1	17.2
Fair value of derivative liabilities presented above	164.7	99.5
Netting of assets and liabilities with the same counterparty	(73.0)	(23.0)
Derivative liability after the effects of netting	91.7	76.5
Spire Missouri
Derivative [Line Items]
Fair value of derivative assets presented above	104.0	6.6
Fair value of cash margin (payable) receivable offset with derivatives	(103.7)	(5.7)
Netting of assets and liabilities with the same counterparty	(0.3)	(0.9)
Derivative asset after the effects of netting	0.0	0.0
Fair value of derivative liabilities presented above	0.3	0.9
Netting of assets and liabilities with the same counterparty	(0.3)	(0.9)
Derivative liability after the effects of netting	$ 0.0	$ 0.0