Fair Value Measurements (Fair Value and Cost of Non-Derivative Instruments) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Carrying Value [Member]
Total Assets	$ 6.4	$ 6.8
Carrying Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Assets	6.4	6.8
Carrying Value [Member] \| Money Market Funds [Member]
Total Master Trust Assets, Cost	0.3	0.3
Carrying Value [Member] \| Money Market Funds [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.3	0.3
Carrying Value [Member] \| Equity Securities [Member]
Total Master Trust Assets, Cost	2.1	2.3
Carrying Value [Member] \| Equity Securities [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	2.1	2.3
Carrying Value [Member] \| Debt Securities [Member]
Total Master Trust Assets, Cost	4.0	4.0
Carrying Value [Member] \| Debt Securities [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	4.0	4.0
Carrying Value [Member] \| Hedge Funds [Member]
Total Master Trust Assets, Cost	0.0	0.1
Carrying Value [Member] \| Hedge Funds [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.0	0.1
Carrying Value [Member] \| Tangible Assets [Member]
Total Master Trust Assets, Cost	0.0	0.1
Carrying Value [Member] \| Tangible Assets [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.0	0.1
Carrying Value [Member] \| Debt [Member]
Long-term Debt	1,393.6	1,363.1
Carrying Value [Member] \| Debt [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Long-term Debt	574.1	574.4
Fair Value [Member]
Total Master Trust Assets, Fair Value	8.9	8.8
Total Assets	8.9	8.8
Fair Value [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.9	8.8
Total Assets	8.9	8.8
Fair Value [Member] \| Money Market Funds [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Fair Value [Member] \| Money Market Funds [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Fair Value [Member] \| Equity Securities [Member]
Total Master Trust Assets, Fair Value	4.5	4.2
Fair Value [Member] \| Equity Securities [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.5	4.2
Fair Value [Member] \| Debt Securities [Member]
Total Master Trust Assets, Fair Value	4.1	4.1
Fair Value [Member] \| Debt Securities [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.1	4.1
Fair Value [Member] \| Hedge Funds [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Fair Value [Member] \| Hedge Funds [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Fair Value [Member] \| Tangible Assets [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Fair Value [Member] \| Tangible Assets [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Fair Value [Member] \| Debt [Member]
Debt, Fair Value	1,571.6	1,404.0
Fair Value [Member] \| Debt [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt, Fair Value	$ 656.0	$ 600.5

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

No definition available.

Name:	dpl_InvestmentsOtherThanDerivatives
Namespace Prefix:	dpl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. 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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120398452&loc=d3e534808-122878

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

+ Details

Name:	us-gaap_OtherInvestmentsAndSecuritiesAtCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=srt_SubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_HedgeFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=dpl_TangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_DebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: