Derivative Instruments and Hedging Activities - Fair Value of Derivative Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	$ 49.2	$ 24.2
Fair value of derivative liabilities presented above	99.5	78.5
Derivatives in Cash Flow Hedging Relationships
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	0.0	0.0
Fair value of derivative liabilities presented above	54.2	43.4
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	49.2	24.2
Fair value of derivative liabilities presented above	45.3	35.1
Gasoline and Heating Oil Contracts | Not Designated as Hedging Instrument | Derivative Instruments Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	0.3
Fair value of derivative liabilities presented above	0.0
Other | Interest rate swaps | Derivatives in Cash Flow Hedging Relationships | Derivative Instrument Liability
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	0.0
Fair value of derivative liabilities presented above	54.2
Other | Interest rate swaps | Derivatives in Cash Flow Hedging Relationships | Derivative Instruments Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above		0.0
Fair value of derivative liabilities presented above		43.4
Gas Utility | Operating Segments | Natural Gas Contracts | Not Designated as Hedging Instrument | Accounts Receivable – Other
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	6.3	0.0
Fair value of derivative liabilities presented above	0.9	12.3
Gas Marketing | Operating Segments | NYMEX / ICE natural gas contracts | Not Designated as Hedging Instrument | Derivative Instruments Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	20.5	7.0
Fair value of derivative liabilities presented above	21.3	7.3
Gas Marketing | Operating Segments | NYMEX / ICE natural gas contracts | Not Designated as Hedging Instrument | Deferred Charges – Other
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	7.2	0.5
Fair value of derivative liabilities presented above	0.8	1.6
Gas Marketing | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Derivative Instruments Assets
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	13.5	10.4
Fair value of derivative liabilities presented above	0.0	0.9
Gas Marketing | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Deferred Charges – Other
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	1.4	4.8
Fair value of derivative liabilities presented above	0.0	0.0
Gas Marketing | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Current Liabilities – Other
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	0.0	1.4
Fair value of derivative liabilities presented above	16.8	11.5
Gas Marketing | Operating Segments | Natural gas commodity contracts | Not Designated as Hedging Instrument | Deferred Credits – Other
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	0.0	0.1
Fair value of derivative liabilities presented above	$ 5.5	$ 1.5