Derivative Instruments and Hedging Activities - Effect of Derivative Instruments on the Statement of Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
Net hedging gain (loss) arising during the period	$ (46.4)	$ 3.9	$ 11.5
Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships
Derivative [Line Items]
Effective portion of gain (loss) reclassified from AOCI to income	1.3	1.5	0.0
Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Other Comprehensive Income (Loss)
Derivative [Line Items]
Net hedging gain (loss) arising during the period	(46.4)	3.9	11.5
Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Other Comprehensive Income (Loss) | Gasoline and Heating Oil Contracts
Derivative [Line Items]
Net hedging gain (loss) arising during the period	0.0	0.0	0.1
Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Gas Utility Other Operating Expenses | Gasoline and Heating Oil Contracts
Derivative [Line Items]
Effective portion of gain (loss) reclassified from AOCI to income	0.0	0.1	0.2
Spire Missouri | Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Other Comprehensive Income (Loss) | Gasoline and Heating Oil Contracts
Derivative [Line Items]
Net hedging gain (loss) arising during the period	0.0	0.0	0.1
Spire Missouri | Cash Flow Hedging | Derivatives in Cash Flow Hedging Relationships | Gas Utility Other Operating Expenses | Gasoline and Heating Oil Contracts
Derivative [Line Items]
Effective portion of gain (loss) reclassified from AOCI to income	$ 0.0	$ 0.1	$ 0.2