Derivative Instruments and Hedging Activities - Effect of Derivative Instruments on Statements of Consolidated Income and Statements of Consolidated Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Derivatives in Cash Flow Hedging Relationships [Abstract]
Net hedging gain (loss) arising during the period	$ (8.9)	$ (46.4)	$ 3.9
Derivatives in Cash Flow Hedging Relationships | Cash Flow Hedging
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	(3.2)	1.3	1.5
Derivatives in Cash Flow Hedging Relationships | Cash Flow Hedging | Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Net hedging gain (loss) arising during the period	(8.9)	(46.4)	3.9
Derivatives in Cash Flow Hedging Relationships | Cash Flow Hedging | Interest rate swaps | Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Net hedging gain (loss) arising during the period	(8.9)	(46.4)	3.9
Derivatives in Cash Flow Hedging Relationships | Cash Flow Hedging | Interest rate swaps | Interest Expense
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	(3.2)	1.3	1.4
Derivatives in Cash Flow Hedging Relationships | Cash Flow Hedging | Gasoline and Heating Oil Contracts | Gas Utility Other Operating Expenses
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	0.0	0.0	0.1
Not Designated as Hedging Instrument
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	(2.6)	(5.9)	2.1
Not Designated as Hedging Instrument | Natural gas commodity contracts | Non Regulated Gas Marketing Operating Revenue
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	9.2	2.5	10.2
Not Designated as Hedging Instrument | Natural gas commodity contracts | Non Regulated Gas Marketing Operating Expenses
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	0.0	(8.4)	(8.1)
Not Designated as Hedging Instrument | NYMEX / ICE Natural Gas Contracts | Non Regulated Gas Marketing Operating Revenue
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	$ (11.8)	$ 0.0	$ 0.0