Fair Value Measurements (Fair Value and Cost of Non-Derivative Instruments) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Hedge Funds [Member]
Total Master Trust Assets, Fair Value		$ 0.0
Carrying Value [Member]
Total Assets	$ 7.6	8.3
Carrying Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Assets	7.6	8.3
Carrying Value [Member] \| Money Market Funds [Member]
Total Master Trust Assets, Cost	0.4	0.2
Carrying Value [Member] \| Money Market Funds [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.4	0.2
Carrying Value [Member] \| Equity Securities [Member]
Total Master Trust Assets, Cost	2.4	3.0
Carrying Value [Member] \| Equity Securities [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	2.4	3.0
Carrying Value [Member] \| Debt Securities [Member]
Total Master Trust Assets, Cost	4.4	4.4
Carrying Value [Member] \| Debt Securities [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	4.4	4.4
Carrying Value [Member] \| Hedge Funds [Member]
Total Master Trust Assets, Cost	0.0	0.4
Carrying Value [Member] \| Hedge Funds [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.0	0.4
Carrying Value [Member] \| Real Estate Funds [Member]
Total Master Trust Assets, Cost	0.3	0.3
Carrying Value [Member] \| Real Estate Funds [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.3	0.3
Carrying Value [Member] \| Tangible Assets [Member]
Total Master Trust Assets, Cost	0.1	0.0
Carrying Value [Member] \| Tangible Assets [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.1	0.0
Carrying Value [Member] \| Debt [Member]
Long-term Debt	1,858.4	1,993.3
Carrying Value [Member] \| Debt [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Long-term Debt	749.4	756.7
Fair Value [Member]
Total Master Trust Assets, Fair Value	8.7	9.0
Total Assets	8.7	9.0
Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.7	9.0
Total Assets	8.7	9.0
Fair Value [Member] \| Money Market Funds [Member]
Total Master Trust Assets, Fair Value	0.4	0.2
Fair Value [Member] \| Money Market Funds [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.4	0.2
Fair Value [Member] \| Equity Securities [Member]
Total Master Trust Assets, Fair Value	3.4	3.8
Fair Value [Member] \| Equity Securities [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.4	3.8
Fair Value [Member] \| Debt Securities [Member]
Total Master Trust Assets, Fair Value	4.4	4.3
Fair Value [Member] \| Debt Securities [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.4	4.3
Fair Value [Member] \| Hedge Funds [Member]
Total Master Trust Assets, Fair Value	0.1	0.4
Fair Value [Member] \| Hedge Funds [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.4
Fair Value [Member] \| Real Estate Funds [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Fair Value [Member] \| Real Estate Funds [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Fair Value [Member] \| Tangible Assets [Member]
Total Master Trust Assets, Fair Value	0.1	0.0
Fair Value [Member] \| Tangible Assets [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.0
Fair Value [Member] \| Debt [Member]
Debt, Fair Value	1,907.7	1,975.3
Fair Value [Member] \| Debt [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt, Fair Value	$ 763.5	$ 764.2

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

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20, 22
-Article 5

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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.

+ Details

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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

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-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph a
-Subparagraph 1(g)
-Article 7

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