Fair Value Measurements (Fair Value and Cost of Non-Derivative Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Cost [Member]
Total Assets	$ 8.4	$ 8.7
Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Assets	8.4	8.7
Fair Value [Member]
Total Master Trust Assets, Fair Value	9.0	9.7
Total Assets	9.0	9.7
Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	9.0	9.7
Total Assets	9.0	9.7
Money Market Funds [Member] \| Cost [Member]
Total Master Trust Assets, Cost	0.2	0.1
Money Market Funds [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.2	0.1
Money Market Funds [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	0.2	0.1
Money Market Funds [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.2	0.1
Equity Securities [Member] \| Cost [Member]
Total Master Trust Assets, Cost	2.7	2.7
Equity Securities [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	2.7	2.7
Equity Securities [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	3.4	3.7
Equity Securities [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.4	3.7
Debt Securities [Member] \| Cost [Member]
Total Master Trust Assets, Cost	4.5	4.7
Debt Securities [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	4.5	4.7
Debt Securities [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	4.4	4.7
Debt Securities [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.4	4.7
Hedge Funds [Member] \| Cost [Member]
Total Master Trust Assets, Cost	0.7	0.8
Hedge Funds [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.7	0.8
Hedge Funds [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	0.7	0.8
Hedge Funds [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.7	0.8
Real Estate Funds [Member] \| Cost [Member]
Total Master Trust Assets, Cost	0.3	0.4
Real Estate Funds [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.3	0.4
Real Estate Funds [Member] \| Fair Value [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Real Estate Funds [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Debt [Member] \| Cost [Member]
Debt, Cost	2,009.4	2,159.7
Debt [Member] \| Cost [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt, Cost	762.9	877.1
Debt [Member] \| Fair Value [Member]
Debt, Fair Value	2,033.7	2,204.8
Debt [Member] \| Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt, Fair Value	$ 764.3	$ 882.5

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

No definition available.

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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/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph a, f, g

-Article 7

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- Definition

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Line-of-Credit Arrangement

-URI http://asc.fasb.org/extlink&oid=6517033

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- Definition

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

No definition available.

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- Definition

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://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.1)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph a

-Subparagraph 1(g)

-Article 7

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