Debt (Long-term Debt) (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Debt to EBITDA Ratio	6.75
Unamortized deferred finance costs	$ (5.5)	$ (5.9)
Unamortized debt discounts and premiums, net	(1.0)	(1.0)
Total long-term debt	1,363.6	1,363.1
Less: current portion	(139.9)	(139.8)
Long-term debt	1,223.7	1,223.3
Long-term Line of Credit	$ 94.0	104.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum	0.65
Long Term Indebtedness, Less than or Equal to	$ 750.0
Unamortized deferred finance costs	(5.3)	(5.4)
Unamortized deferred finance costs	(5.3)	(5.4)
Unamortized debt discounts and premiums, net	(2.7)	(2.7)
Unamortized debt discounts and premiums, net	(2.7)	(2.7)
Total long-term debt at subsidiary	574.5	574.4
Total long-term debt	574.5	574.4
Less: current portion	(139.9)	(139.8)
Long-term debt	434.6	434.6
Long-term Line of Credit	$ 85.0	40.0
Term Loan Maturing 2022 [Domain]
Debt Instrument [Line Items]
Debt instrument maturity year	Jan. 01, 2049
Term Loan Maturing 2022 [Domain] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt instrument maturity year	Jan. 01, 2049
3.95% Senior Notes due 2049 [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 425.0	425.0
Debt instrument maturity year	Aug. 01, 2020
Pollution Control Series Maturing in August 2020 - 1.13% - 1.14% [Member]
Debt Instrument [Line Items]
Debt instrument maturity year	Aug. 01, 2020
Pollution Control Series Maturing in August 2020 - 1.13% - 1.14% [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 140.0	140.0
U.S. Government note maturing in February 2061 - 4.20% [Member]
Debt Instrument [Line Items]
Debt instrument maturity year	Feb. 01, 2061
Debt instrument interest percentage	4.20%
U.S. Government note maturing in February 2061 - 4.20% [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 17.5	17.5
Senior unsecured due in October 2021 - 7.25% [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 380.0	380.0
Debt instrument maturity year	Oct. 01, 2021
Debt instrument interest percentage	7.25%
4.35% Senior Notes due 2029 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 400.0	400.0
Debt instrument maturity year	Apr. 15, 2029
Debt instrument interest percentage	4.35%
Note to DPL Capital Trust II Maturing in September 2031 - 8.125% [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 15.6	$ 15.6
Debt instrument maturity year	Sep. 01, 2031
Debt instrument interest percentage	8.125%
Maximum [Member] \| Term Loan Maturing 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage
Debt Instrument, Interest Rate, Effective Percentage
Maximum [Member] \| Pollution Control Series Maturing in August 2020 - 1.13% - 1.14% [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage		1.42%
Maximum [Member] \| Zero Point Two Three To Zero Point Two Nine And Zero Point One Six To Zero Point Three Six Percentage Of Bonds Maturing In November Two Thousand Forty [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage	2.93%
Minimum [Member] \| Term Loan Maturing 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage
Debt Instrument, Interest Rate, Effective Percentage	3.95%
Minimum [Member] \| Pollution Control Series Maturing in August 2020 - 1.13% - 1.14% [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage	4.20%	1.29%
Minimum [Member] \| Zero Point Two Three To Zero Point Two Nine And Zero Point One Six To Zero Point Three Six Percentage Of Bonds Maturing In November Two Thousand Forty [Member]
Debt Instrument [Line Items]
Debt instrument maturity year	Feb. 01, 2061
Debt instrument interest percentage	2.40%

Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum

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Debt to EBITDA Ratio

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Deferred Finance Costs, Net, Including Acquisition Adjustments

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After separation required compliance with the financial covenant, shall be suspended from 1/1/17 to 12/31/17 if during such time DP&L's LT indebtedness (as determined by the PUCO) is less than or equal to $750.0 million.

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Total Long-term Debt At Subsidiary With Purchase Accounting Adjustments

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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Amount, after accumulated amortization, of debt discount.

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Amount, after accumulated amortization, of debt discount (premium).

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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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.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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