Derivative Instruments and Hedging Activities (Gains or Losses Recognized in AOCI for the Cash Flow Hedges) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	$ (2.8)	$ (4.3)
Net gains / (losses) associated with current period hedging transactions	0.0	4.9
Ending accumulated derivative gain / (loss) in AOCI	(0.1)	1.8
Portion expected to be reclassified to earnings in the next twelve months	$ 0.0
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	0 months
Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	$ 17.5	17.4
Net gains / (losses) associated with current period hedging transactions	0.9	0.3
Ending accumulated derivative gain / (loss) in AOCI	18.0	17.5
Portion expected to be reclassified to earnings in the next twelve months	$ (0.4)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	29 months
THE DAYTON POWER AND LIGHT COMPANY [Member] \| Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	$ 0.0	(4.3)
Net gains / (losses) associated with current period hedging transactions	0.0	0.0
Ending accumulated derivative gain / (loss) in AOCI	0.0	1.8
Portion expected to be reclassified to earnings in the next twelve months	$ 0.0
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	0 months
THE DAYTON POWER AND LIGHT COMPANY [Member] \| Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	$ 1.4	1.6
Net gains / (losses) associated with current period hedging transactions	0.5	0.3
Ending accumulated derivative gain / (loss) in AOCI	1.6	1.7
Portion expected to be reclassified to earnings in the next twelve months	$ (0.3)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	29 months
Interest Expense [Member] \| Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	$ 0.0	0.0
Interest Expense [Member] \| Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	(0.4)	(0.2)
Interest Expense [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member] \| Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.0	0.0
Interest Expense [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member] \| Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	(0.3)	(0.2)
Discontinued Operations, Disposed of by Means Other than Sale, Spinoff [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member] \| Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.0	6.1
Discontinued Operations, Disposed of by Means Other than Sale, Spinoff [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member] \| Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.0	0.0
Revenue [Member] \| Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	4.1	(0.9)
Revenue [Member] \| Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.0	0.0
Purchased Power [Member] \| Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	(1.4)	2.1
Purchased Power [Member] \| Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	$ 0.0	$ 0.0

Accumulated Derivative Gain/Loss in AOCI [Roll Forward]

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- Definition

Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 220
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-Section 45
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80784-113994

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- Definition

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

+ References

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- Definition

The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
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- Definition

Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
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