Derivative Instruments and Hedging Activities - Fair Value of Derivative Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	$ 24.2	$ 27.4
Fair value of derivative liabilities presented above	78.5	21.0
Derivatives in Cash Flow Hedging Relationships
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	0.0	3.0
Fair value of derivative liabilities presented above	43.4	0.0
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative assets presented above	24.2	24.4
Fair value of derivative liabilities presented above	35.1	21.0
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	43.4
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		3.0
Fair value of derivative liabilities presented above		0.0
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	0.0	2.7
Fair value of derivative liabilities presented above	12.3	1.9
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	7.0	3.8
Fair value of derivative liabilities presented above	7.3	10.7
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	0.5	0.4
Fair value of derivative liabilities presented above	1.6	0.7
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	10.4	10.9
Fair value of derivative liabilities presented above	0.9	1.0
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	4.8	6.3
Fair value of derivative liabilities presented above	0.0	0.2
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	1.4	0.3
Fair value of derivative liabilities presented above	11.5	6.3
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	0.1	0.0
Fair value of derivative liabilities presented above	$ 1.5	$ 0.2