Fair Value (Fair Value and Cost of Non-Derivative Instruments) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Sep. 30, 2018
Unrealized Gains and Immaterial Unrealized Losses in AOCI, Before Tax	$ 1.6
THE DAYTON POWER AND LIGHT COMPANY [Member]
Unrealized Gains and Immaterial Unrealized Losses in AOCI, Before Tax	1.7
Cost [Member]
Total Assets	7.3	$ 7.1
Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Assets	7.3	7.1
Fair Value [Member]
Total Master Trust Assets, Fair Value	9.1	8.7
Total Assets	9.1	8.7
Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	9.1	8.7
Total Assets	9.1	8.7
Money Market Funds [Member] \| Cost [Member]
Total Master Trust Assets, Cost	0.3	0.3
Money Market Funds [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.3	0.3
Money Market Funds [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Money Market Funds [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Equity Securities [Member] \| Cost [Member]
Total Master Trust Assets, Cost	2.5	2.4
Equity Securities [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	2.5	2.4
Equity Securities [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	4.2	4.1
Equity Securities [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.2	4.1
Debt Securities [Member] \| Cost [Member]
Total Master Trust Assets, Cost	4.3	4.1
Debt Securities [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	4.3	4.1
Debt Securities [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	4.3	4.0
Debt Securities [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.3	4.0
Hedge Funds [Member] \| Cost [Member]
Total Master Trust Assets, Cost	0.1	0.2
Hedge Funds [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.1	0.2
Hedge Funds [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	0.2	0.2
Hedge Funds [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.2	0.2
Tangible Assets [Member] \| Cost [Member]
Total Master Trust Assets, Cost	0.1	0.1
Tangible Assets [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.1	0.1
Tangible Assets [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Tangible Assets [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Debt [Member] \| Cost [Member]
Debt, Cost	1,704.8	1,476.0
Debt [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt, Cost	646.6	586.7
Debt [Member] \| Fair Value [Member]
Debt, Fair Value	1,819.3	1,560.1
Debt [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt, Fair Value	$ 658.4	$ 594.9

The total of assets that are measured on a recurring basis.

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- Definition

Unrealized Gains and Immaterial Unrealized Losses in AOCI, Before Tax

+ References

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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Fair value portion of contractual arrangement with a lender under which borrowings can be made up to a specific amount at any point in time.

+ References

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

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Includes other investments and securities that are not at fair value as of the balance sheet date and those that are not accounted for under the equity method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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-Section S99
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