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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
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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

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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4CC -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109998890-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959 + Details
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