Derivative Instruments and Hedging Activities (Gains or Losses Recognized in AOCI for the Cash Flow Hedges) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Forward Contract Power [Member]
Beginning accumulated derivative gain / (loss) in AOCI	$ (4.3)	$ 9.2	$ 0.2
Net gains / (losses) associated with current period hedging transactions	8.8	15.7	18.2
Ending accumulated derivative gain / (loss) in AOCI	(2.8)	(4.3)	9.2
Portion expected to be reclassified to earnings in the next twelve months	$ (2.7)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	3 months
Interest Rate Contract [Member]
Beginning accumulated derivative gain / (loss) in AOCI	$ 17.4	17.5	18.3
Net gains / (losses) associated with current period hedging transactions	0.8	0.4	0.0
Ending accumulated derivative gain / (loss) in AOCI	17.5	17.4	17.5
Portion expected to be reclassified to earnings in the next twelve months	$ (0.7)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	32 months
THE DAYTON POWER AND LIGHT COMPANY [Member] \| Forward Contract Power [Member]
Derivative Instrument, loss from discontinued operations	$ (5.5)	(29.2)	(9.2)
Beginning accumulated derivative gain / (loss) in AOCI	(4.3)	9.2	0.2
Net gains / (losses) associated with current period hedging transactions	10.7	15.7	18.2
Ending accumulated derivative gain / (loss) in AOCI	(1.2)	(4.3)	9.2
Derivative Instrument, transfer of generation assets of subsidiary	(2.1)
THE DAYTON POWER AND LIGHT COMPANY [Member] \| Interest Rate Contract [Member]
Beginning accumulated derivative gain / (loss) in AOCI	1.6	2.0	2.6
Net gains / (losses) associated with current period hedging transactions	1.7	0.4	0.0
Ending accumulated derivative gain / (loss) in AOCI	2.6	1.6	2.0
Portion expected to be reclassified to earnings in the next twelve months	$ (0.7)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	32 months
Interest Expense [Member] \| Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 0.0	0.0	0.0
Interest Expense [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(0.7)	(0.5)	(0.8)
Interest Expense [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(0.7)	(0.8)	(0.6)
Sales [Member] \| Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(9.8)	(35.6)	(12.0)
Sales [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	0.0	0.0	0.0
Purchased Power [Member] \| Forward Contract Power [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	2.5	6.4	2.8
Purchased Power [Member] \| Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 0.0	$ 0.0	$ 0.0

Derivative Instrument, loss from discontinued operations

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- Definition

Derivative Instrument, transfer of generation assets of subsidiary

+ References

No definition available.

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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
-SubTopic 10
-Section 45
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
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-Section 45
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-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)
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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.

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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
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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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
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