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, 2019	Sep. 30, 2018	Sep. 30, 2017
Derivatives in Cash Flow Hedging Relationships [Abstract]
Net hedging gain (loss) arising during the period	$ (46.4)	$ 3.9	$ 11.5
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	1.3	1.5	0.0
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.0	0.5
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	(46.4)	3.9	11.5
Derivatives in Cash Flow Hedging Relationships | Cash Flow Hedging | Gasoline and Heating Oil Contracts | Other Comprehensive Income (Loss)
Derivatives in Cash Flow Hedging Relationships [Abstract]
Net hedging gain (loss) arising during the period	0.0	0.0	0.1
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	0.1	0.2
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	(46.4)	3.9	11.4
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	1.3	1.4	0.1
Ineffective portion of gain (loss) on derivatives recognized in income	0.0	0.0	0.5
Derivatives in Cash Flow Hedging Relationships | Cash Flow Hedging | Natural Gas Contracts
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	0.0	0.0	(0.3)
Derivatives in Cash Flow Hedging Relationships | Cash Flow Hedging | Natural Gas Contracts | Gas Marketing Operating Revenues
Derivatives in Cash Flow Hedging Relationships [Abstract]
Effective portion of gain (loss) reclassified from AOCI to income	0.0	0.0	(0.4)
Derivatives in Cash Flow Hedging Relationships | Cash Flow Hedging | Natural Gas Contracts | Gas Marketing 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	(5.9)	2.1	(3.7)
Not Designated as Hedging Instrument | Natural gas commodity contracts | Gas Marketing Operating Revenues
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	2.5	10.2	0.7
Not Designated as Hedging Instrument | Natural gas commodity contracts | Gas Marketing Operating Expenses
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	(8.4)	(8.1)	0.0
Not Designated as Hedging Instrument | NYMEX / ICE Natural Gas Contracts | Gas Marketing Operating Revenues
Derivatives Not Designated as Hedging Instruments [Abstract]
Gain (loss) recognized in income on derivatives	$ 0.0	$ 0.0	$ (4.4)