Regulatory Matters (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets, Noncurrent	$ 177.0		$ 173.8
Unbilled Revenue	15.1		19.4
Due from PJM transmission settlement	1.8		1.8
Recovery of Smart Grid Plan Phase 1 capital investments	249.0
Aggregate cap of recoverable Smart Grid Plan investments and expenses	267.6
DP&L shareholder funding under Smart Grid Plan	$ 30.0
Schedule of Regulatory Assets and Liabilities [Text Block]	Regulatory Matters ESP v. MRO and SEET Proceedings Ohio law requires utilities to file either an ESP or MRO plan to establish SSO rates. From November 1, 2017 through December 18, 2019, DP&L operated pursuant to an approved ESP plan, which was initially approved on October 20, 2017 (ESP 3). On November 21, 2019, the PUCO issued a supplemental order modifying ESP 3, and as a result DP&L filed a Notice of Withdrawal of its ESP 3 Application and requested to revert to the ESP rates that were in effect prior to ESP 3. The Notice of Withdrawal was approved by the PUCO on December 18, 2019. The PUCO order required, among other things, DP&L to conduct both an ESP v. MRO Test to validate that the ESP is expected to be more favorable in the aggregate than what would be experienced under an MRO, and a prospective SEET, which were filed with the PUCO on April 1, 2020. DP&L is also subject to an annual retrospective SEET. On October 23, 2020, DP&L entered into a Stipulation and Recommendation (settlement) with the staff of the PUCO and various customers, and organizations representing customers of DP&L and certain other parties with respect to, among other matters, DP&L’s applications pending at the PUCO for (i) approval of DP&L’s plan to modernize its distribution grid (the Smart Grid Plan), (ii) findings that DP&L passed the SEET for 2018 and 2019, and (iii) findings that DP&L’s current ESP 1 satisfies the SEET and the more favorable in the aggregate (MFA) regulatory test. The settlement is subject to, and conditioned upon, approval by the PUCO. A hearing has been set for January 11, 2021 for consideration of this settlement. The settlement would provide, among other items, for the following: •Approval of the Smart Grid Plan outlined in the Smart Grid Plan application filed by DP&L with the PUCO, as modified by the terms of the settlement, including, subject to offsetting operational benefits and certain other conditions, a return on and recovery of up to $249.0 million of Smart Grid Plan Phase 1 capital investments and recovery of operational and maintenance expenses through DP&L’s existing Infrastructure Investment Rider for a term of four years, under an aggregate cap of $267.6 million on the amount of such investments and expenses that is recoverable, and an acknowledgement that DP&L may file a subsequent application with the PUCO within three years seeking approvals for Phase 2 of the Smart Grid Plan; •A commitment by DP&L to invest in a customer information system and supporting technologies during Phase 1 of the Smart Grid Plan, with DP&L recovering a return on and of prudently incurred capital investments and operational and maintenance expenses, including deferred operational and maintenance expense amounts, in a future rate case; •A determination that DP&L’s ESP 1 satisfies the prospective SEET and the MFA regulatory test; •A recommendation by parties to the settlement that the PUCO also finds that DP&L satisfies the retrospective SEET for 2018 and 2019; •A commitment by DP&L to file an application with the PUCO no later than October 1, 2023 for a new electric security plan that does not seek to implement certain non-bypassable charges, including those related to provider of last resort risks, stability, or financial integrity; and •DP&L shareholder funding, in an aggregate amount of approximately $30.0 million over four years, for certain economic development discounts, incentives, and grants to certain commercial and industrial customers, including hospitals and manufacturers, assistance for low-income customers as well as the residents and businesses of the City of Dayton, and promotion of solar and resiliency development within DP&L’s service territory. Certain parties which intervened in the ESP proceedings have filed petitions for rehearing of the recent PUCO ESP orders; some of which seek to eliminate DP&L’s RSC from the ESP 1 rates that are currently in place and others seek to re-implement the 2017 ESP, but without the DMR. We are unable to predict the outcomes of these petitions, but if these result in terms that are more adverse than DP&L's current ESP rate plan, it could have a material adverse effect on our results of operations, financial condition and cash flows. The parties signing the above-referenced settlement have agreed to withdraw their respective petitions if the settlement is approved by the PUCO without material modification. Decoupling On January 23, 2020 DP&L filed with the PUCO requesting approval to defer its decoupling costs consistent with the methodology approved in its Distribution Rate Case. If approved, deferral would be effective December 18, 2019 and going forward would reduce impacts of weather, energy efficiency programs and economic changes in customer demand. COVID-19 In response to the PUCO’s COVID-19 emergency orders, DP&L filed an Application on March 23, 2020, requesting waivers of certain rule and tariff requirements and deferral of certain costs and revenues including those related to deposits and reconnection fees, late payment fees, credit card fees; and waived or uncollected amounts associated with putting customers on payment plans. On May 20, 2020, the PUCO approved the application and required DP&L to file a plan outlining the timing and steps it plans to take in an effort to return to normal operations. The authorized deferral of those certain costs and revenues must be offset by COVID-19 related savings. DP&L filed its plan on July 15, 2020 and was approved by the PUCO on August 12, 2020. As a result, DP&L has recorded a $0.8 million regulatory asset as of September 30, 2020. Recovery of these deferrals will be addressed in a future rate proceeding. FERC Proceedings On November 15, 2018 the FERC issued a Notice of Proposed Rulemaking (NOPR) to address amortization of excess accumulated deferred income taxes resulting from the TCJA and their impact on transmission rates. Such notice requires all public utility transmission providers with stated transmission rates under an Open Access Transmission Tariff (OATT) to determine the amount of excess deferred income taxes caused by the TCJA. On March 3, 2020, DP&L filed an application before the FERC to change its transmission rate from a stated rate to a formula rate, which was accepted by the FERC and made effective as of May 3, 2020, subject to further proceeding and potential refunds. The formula rate includes adjustments to flow back over time the excess deferred income taxes caused by the TCJA. The NOPR, therefore, no longer applies to DP&L. The rate changes will increase revenues by approximately $4.1 million through the end of 2020 as of the effective date, subject to refund based on final approved rates.
Subsidiaries [Member]
Regulatory Assets, Noncurrent	$ 177.0		173.8
Unbilled Revenue	15.1		19.4
Due from PJM transmission settlement	1.8		$ 1.8
Recovery of Smart Grid Plan Phase 1 capital investments	249.0
Aggregate cap of recoverable Smart Grid Plan investments and expenses	267.6
DP&L shareholder funding under Smart Grid Plan	$ 30.0
Schedule of Regulatory Assets and Liabilities [Text Block]	Regulatory Matters ESP v. MRO and SEET Proceedings Ohio law requires utilities to file either an ESP or MRO plan to establish SSO rates. From November 1, 2017 through December 18, 2019, DP&L operated pursuant to an approved ESP plan, which was initially approved on October 20, 2017 (ESP 3). On November 21, 2019, the PUCO issued a supplemental order modifying ESP 3, and as a result DP&L filed a Notice of Withdrawal of its ESP 3 Application and requested to revert to the ESP rates that were in effect prior to ESP 3. The Notice of Withdrawal was approved by the PUCO on December 18, 2019. The PUCO order required, among other things, DP&L to conduct both an ESP v. MRO Test to validate that the ESP is expected to be more favorable in the aggregate than what would be experienced under an MRO, and a prospective SEET, which were filed with the PUCO on April 1, 2020. DP&L is also subject to an annual retrospective SEET. On October 23, 2020, DP&L entered into a Stipulation and Recommendation (settlement) with the staff of the PUCO and various customers, and organizations representing customers of DP&L and certain other parties with respect to, among other matters, DP&L’s applications pending at the PUCO for (i) approval of DP&L’s plan to modernize its distribution grid (the Smart Grid Plan), (ii) findings that DP&L passed the SEET for 2018 and 2019, and (iii) findings that DP&L’s current ESP 1 satisfies the SEET and the more favorable in the aggregate (MFA) regulatory test. The settlement is subject to, and conditioned upon, approval by the PUCO. A hearing has been set for January 11, 2021 for consideration of this settlement. The settlement would provide, among other items, for the following: •Approval of the Smart Grid Plan outlined in the Smart Grid Plan application filed by DP&L with the PUCO, as modified by the terms of the settlement, including, subject to offsetting operational benefits and certain other conditions, a return on and recovery of up to $249.0 million of Smart Grid Plan Phase 1 capital investments and recovery of operational and maintenance expenses through DP&L’s existing Infrastructure Investment Rider for a term of four years, under an aggregate cap of $267.6 million on the amount of such investments and expenses that is recoverable, and an acknowledgement that DP&L may file a subsequent application with the PUCO within three years seeking approvals for Phase 2 of the Smart Grid Plan; •A commitment by DP&L to invest in a customer information system and supporting technologies during Phase 1 of the Smart Grid Plan, with DP&L recovering a return on and of prudently incurred capital investments and operational and maintenance expenses, including deferred operational and maintenance expense amounts, in a future rate case; •A determination that DP&L’s ESP 1 satisfies the prospective SEET and the MFA regulatory test; •A recommendation by parties to the settlement that the PUCO also finds that DP&L satisfies the retrospective SEET for 2018 and 2019; •A commitment by DP&L to file an application with the PUCO no later than October 1, 2023 for a new electric security plan that does not seek to implement certain non-bypassable charges, including those related to provider of last resort risks, stability, or financial integrity; and •DP&L shareholder funding, in an aggregate amount of approximately $30.0 million over four years, for certain economic development discounts, incentives, and grants to certain commercial and industrial customers, including hospitals and manufacturers, assistance for low-income customers as well as the residents and businesses of the City of Dayton, and promotion of solar and resiliency development within DP&L’s service territory. Certain parties which intervened in the ESP proceedings have filed petitions for rehearing of the recent PUCO ESP orders; some of which seek to eliminate DP&L’s RSC from the ESP 1 rates that are currently in place and others seek to re-implement the 2017 ESP, but without the DMR. We are unable to predict the outcomes of these petitions, but if these result in terms that are more adverse than DP&L's current ESP rate plan, it could have a material adverse effect on our results of operations, financial condition and cash flows. The parties signing the above-referenced settlement have agreed to withdraw their respective petitions if the settlement is approved by the PUCO without material modification. Decoupling On January 23, 2020 DP&L filed with the PUCO requesting approval to defer its decoupling costs consistent with the methodology approved in its Distribution Rate Case. If approved, deferral would be effective December 18, 2019 and going forward would reduce impacts of weather, energy efficiency programs and economic changes in customer demand. COVID-19 In response to the PUCO’s COVID-19 emergency orders, DP&L filed an Application on March 23, 2020, requesting waivers of certain rule and tariff requirements and deferral of certain costs and revenues including those related to deposits and reconnection fees, late payment fees, credit card fees; and waived or uncollected amounts associated with putting customers on payment plans. On May 20, 2020, the PUCO approved the application and required DP&L to file a plan outlining the timing and steps it plans to take in an effort to return to normal operations. The authorized deferral of those certain costs and revenues must be offset by COVID-19 related savings. DP&L filed its plan on July 15, 2020 and was approved by the PUCO on August 12, 2020. As a result, DP&L has recorded a $0.8 million regulatory asset as of September 30, 2020. Recovery of these deferrals will be addressed in a future rate proceeding. FERC Proceedings On November 15, 2018 the FERC issued a Notice of Proposed Rulemaking (NOPR) to address amortization of excess accumulated deferred income taxes resulting from the TCJA and their impact on transmission rates. Such notice requires all public utility transmission providers with stated transmission rates under an Open Access Transmission Tariff (OATT) to determine the amount of excess deferred income taxes caused by the TCJA. On March 3, 2020, DP&L filed an application before the FERC to change its transmission rate from a stated rate to a formula rate, which was accepted by the FERC and made effective as of May 3, 2020, subject to further proceeding and potential refunds. The formula rate includes adjustments to flow back over time the excess deferred income taxes caused by the TCJA. The NOPR, therefore, no longer applies to DP&L. The rate changes will increase revenues by approximately $4.1 million through the end of 2020 as of the effective date, subject to refund based on final approved rates.
COVID-19 Deferral [Member]
Regulatory Assets, Noncurrent	$ 0.8
COVID-19 Deferral [Member] \| Subsidiaries [Member]
Regulatory Assets, Noncurrent	$ 0.8
Forecast [Member]
Increase in revenues from proposed transmission rate change to a formula rate		$ 4.1
Forecast [Member] \| Subsidiaries [Member]
Increase in revenues from proposed transmission rate change to a formula rate		$ 4.1

$ 177.0

$ 173.8

1.8

Aggregate cap of recoverable Smart Grid Plan investments and expenses

267.6

Schedule of Regulatory Assets and Liabilities [Text Block]

Regulatory Matters

ESP v. MRO and SEET Proceedings

Ohio law requires utilities to file either an ESP or MRO plan to establish SSO rates. From November 1, 2017 through December 18, 2019, DP&L operated pursuant to an approved ESP plan, which was initially approved on October 20, 2017 (ESP 3). On November 21, 2019, the PUCO issued a supplemental order modifying ESP 3, and as a result DP&L filed a Notice of Withdrawal of its ESP 3 Application and requested to revert to the ESP rates that were in effect prior to ESP 3. The Notice of Withdrawal was approved by the PUCO on December 18, 2019. The PUCO order required, among other things, DP&L to conduct both an ESP v. MRO Test to validate that the ESP is expected to be more favorable in the aggregate than what would be experienced under an MRO, and a prospective SEET, which were filed with the PUCO on April 1, 2020. DP&L is also subject to an annual retrospective SEET.

On October 23, 2020, DP&L entered into a Stipulation and Recommendation (settlement) with the staff of the PUCO and various customers, and organizations representing customers of DP&L and certain other parties with respect to, among other matters, DP&L’s applications pending at the PUCO for (i) approval of DP&L’s plan to modernize its distribution grid (the Smart Grid Plan), (ii) findings that DP&L passed the SEET for 2018 and 2019, and (iii) findings that DP&L’s current ESP 1 satisfies the SEET and the more favorable in the aggregate (MFA) regulatory test. The settlement is subject to, and conditioned upon, approval by the PUCO. A hearing has been set for January 11, 2021 for consideration of this settlement. The settlement would provide, among other items, for the following:

•Approval of the Smart Grid Plan outlined in the Smart Grid Plan application filed by DP&L with the PUCO, as modified by the terms of the settlement, including, subject to offsetting operational benefits and certain

other conditions, a return on and recovery of up to $249.0 million of Smart Grid Plan Phase 1 capital investments and recovery of operational and maintenance expenses through DP&L’s existing Infrastructure Investment Rider for a term of four years, under an aggregate cap of $267.6 million on the amount of such investments and expenses that is recoverable, and an acknowledgement that DP&L may file a subsequent application with the PUCO within three years seeking approvals for Phase 2 of the Smart Grid Plan;

•A commitment by DP&L to invest in a customer information system and supporting technologies during Phase 1 of the Smart Grid Plan, with DP&L recovering a return on and of prudently incurred capital investments and operational and maintenance expenses, including deferred operational and maintenance expense amounts, in a future rate case;

•A determination that DP&L’s ESP 1 satisfies the prospective SEET and the MFA regulatory test;

•A recommendation by parties to the settlement that the PUCO also finds that DP&L satisfies the retrospective SEET for 2018 and 2019;

•A commitment by DP&L to file an application with the PUCO no later than October 1, 2023 for a new electric security plan that does not seek to implement certain non-bypassable charges, including those related to provider of last resort risks, stability, or financial integrity; and

•DP&L shareholder funding, in an aggregate amount of approximately $30.0 million over four years, for certain economic development discounts, incentives, and grants to certain commercial and industrial customers, including hospitals and manufacturers, assistance for low-income customers as well as the residents and businesses of the City of Dayton, and promotion of solar and resiliency development within DP&L’s service territory.

Certain parties which intervened in the ESP proceedings have filed petitions for rehearing of the recent PUCO ESP orders; some of which seek to eliminate DP&L’s RSC from the ESP 1 rates that are currently in place and others seek to re-implement the 2017 ESP, but without the DMR. We are unable to predict the outcomes of these petitions, but if these result in terms that are more adverse than DP&L's current ESP rate plan, it could have a material adverse effect on our results of operations, financial condition and cash flows. The parties signing the above-referenced settlement have agreed to withdraw their respective petitions if the settlement is approved by the PUCO without material modification.

Decoupling

On January 23, 2020 DP&L filed with the PUCO requesting approval to defer its decoupling costs consistent with the methodology approved in its Distribution Rate Case. If approved, deferral would be effective December 18, 2019 and going forward would reduce impacts of weather, energy efficiency programs and economic changes in customer demand.

COVID-19

In response to the PUCO’s COVID-19 emergency orders, DP&L filed an Application on March 23, 2020, requesting waivers of certain rule and tariff requirements and deferral of certain costs and revenues including those related to deposits and reconnection fees, late payment fees, credit card fees; and waived or uncollected amounts associated with putting customers on payment plans. On May 20, 2020, the PUCO approved the application and required DP&L to file a plan outlining the timing and steps it plans to take in an effort to return to normal operations. The authorized deferral of those certain costs and revenues must be offset by COVID-19 related savings. DP&L filed its plan on July 15, 2020 and was approved by the PUCO on August 12, 2020. As a result, DP&L has recorded a

$0.8 million regulatory asset as of September 30, 2020. Recovery of these deferrals will be addressed in a future

rate proceeding.

FERC Proceedings

On November 15, 2018 the FERC issued a Notice of Proposed Rulemaking (NOPR) to address amortization of excess accumulated deferred income taxes resulting from the TCJA and their impact on transmission rates. Such notice requires all public utility transmission providers with stated transmission rates under an Open Access Transmission Tariff (OATT) to determine the amount of excess deferred income taxes caused by the TCJA. On March 3, 2020, DP&L filed an application before the FERC to change its transmission rate from a stated rate to a formula rate, which was accepted by the FERC and made effective as of May 3, 2020, subject to further proceeding and potential refunds. The formula rate includes adjustments to flow back over time the excess deferred income taxes caused by the TCJA. The NOPR, therefore, no longer applies to DP&L. The rate changes will increase revenues by approximately $4.1 million through the end of 2020 as of the effective date, subject to refund based on final approved rates.

Regulatory Assets, Noncurrent

$ 177.0

173.8

Due from PJM transmission settlement

1.8

$ 1.8

Aggregate cap of recoverable Smart Grid Plan investments and expenses

267.6

Schedule of Regulatory Assets and Liabilities [Text Block]

Regulatory Matters

ESP v. MRO and SEET Proceedings

Ohio law requires utilities to file either an ESP or MRO plan to establish SSO rates. From November 1, 2017 through December 18, 2019, DP&L operated pursuant to an approved ESP plan, which was initially approved on October 20, 2017 (ESP 3). On November 21, 2019, the PUCO issued a supplemental order modifying ESP 3, and as a result DP&L filed a Notice of Withdrawal of its ESP 3 Application and requested to revert to the ESP rates that were in effect prior to ESP 3. The Notice of Withdrawal was approved by the PUCO on December 18, 2019. The PUCO order required, among other things, DP&L to conduct both an ESP v. MRO Test to validate that the ESP is expected to be more favorable in the aggregate than what would be experienced under an MRO, and a prospective SEET, which were filed with the PUCO on April 1, 2020. DP&L is also subject to an annual retrospective SEET.

On October 23, 2020, DP&L entered into a Stipulation and Recommendation (settlement) with the staff of the PUCO and various customers, and organizations representing customers of DP&L and certain other parties with respect to, among other matters, DP&L’s applications pending at the PUCO for (i) approval of DP&L’s plan to modernize its distribution grid (the Smart Grid Plan), (ii) findings that DP&L passed the SEET for 2018 and 2019, and (iii) findings that DP&L’s current ESP 1 satisfies the SEET and the more favorable in the aggregate (MFA) regulatory test. The settlement is subject to, and conditioned upon, approval by the PUCO. A hearing has been set for January 11, 2021 for consideration of this settlement. The settlement would provide, among other items, for the following:

•Approval of the Smart Grid Plan outlined in the Smart Grid Plan application filed by DP&L with the PUCO, as modified by the terms of the settlement, including, subject to offsetting operational benefits and certain other conditions, a return on and recovery of up to $249.0 million of Smart Grid Plan Phase 1 capital investments and recovery of operational and maintenance expenses through DP&L’s existing Infrastructure Investment Rider for a term of four years, under an aggregate cap of $267.6 million on the amount of such

investments and expenses that is recoverable, and an acknowledgement that DP&L may file a subsequent application with the PUCO within three years seeking approvals for Phase 2 of the Smart Grid Plan;

•A commitment by DP&L to invest in a customer information system and supporting technologies during Phase 1 of the Smart Grid Plan, with DP&L recovering a return on and of prudently incurred capital investments and operational and maintenance expenses, including deferred operational and maintenance expense amounts, in a future rate case;

•A determination that DP&L’s ESP 1 satisfies the prospective SEET and the MFA regulatory test;

•A recommendation by parties to the settlement that the PUCO also finds that DP&L satisfies the retrospective SEET for 2018 and 2019;

•A commitment by DP&L to file an application with the PUCO no later than October 1, 2023 for a new electric security plan that does not seek to implement certain non-bypassable charges, including those related to provider of last resort risks, stability, or financial integrity; and

•DP&L shareholder funding, in an aggregate amount of approximately $30.0 million over four years, for certain economic development discounts, incentives, and grants to certain commercial and industrial customers, including hospitals and manufacturers, assistance for low-income customers as well as the residents and businesses of the City of Dayton, and promotion of solar and resiliency development within DP&L’s service territory.

Certain parties which intervened in the ESP proceedings have filed petitions for rehearing of the recent PUCO ESP orders; some of which seek to eliminate DP&L’s RSC from the ESP 1 rates that are currently in place and others seek to re-implement the 2017 ESP, but without the DMR. We are unable to predict the outcomes of these petitions, but if these result in terms that are more adverse than DP&L's current ESP rate plan, it could have a material adverse effect on our results of operations, financial condition and cash flows. The parties signing the above-referenced settlement have agreed to withdraw their respective petitions if the settlement is approved by the PUCO without material modification.

Decoupling

On January 23, 2020 DP&L filed with the PUCO requesting approval to defer its decoupling costs consistent with the methodology approved in its Distribution Rate Case. If approved, deferral would be effective December 18, 2019 and going forward would reduce impacts of weather, energy efficiency programs and economic changes in customer demand.

COVID-19

In response to the PUCO’s COVID-19 emergency orders, DP&L filed an Application on March 23, 2020, requesting waivers of certain rule and tariff requirements and deferral of certain costs and revenues including those related to deposits and reconnection fees, late payment fees, credit card fees; and waived or uncollected amounts associated with putting customers on payment plans. On May 20, 2020, the PUCO approved the application and required DP&L to file a plan outlining the timing and steps it plans to take in an effort to return to normal operations. The authorized deferral of those certain costs and revenues must be offset by COVID-19 related savings. DP&L filed its plan on July 15, 2020 and was approved by the PUCO on August 12, 2020. As a result, DP&L has recorded a

$0.8 million regulatory asset as of September 30, 2020. Recovery of these deferrals will be addressed in a future

rate proceeding.

FERC Proceedings

On November 15, 2018 the FERC issued a Notice of Proposed Rulemaking (NOPR) to address amortization of excess accumulated deferred income taxes resulting from the TCJA and their impact on transmission rates. Such notice requires all public utility transmission providers with stated transmission rates under an Open Access Transmission Tariff (OATT) to determine the amount of excess deferred income taxes caused by the TCJA. On March 3, 2020, DP&L filed an application before the FERC to change its transmission rate from a stated rate to a formula rate, which was accepted by the FERC and made effective as of May 3, 2020, subject to further proceeding and potential refunds. The formula rate includes adjustments to flow back over time the excess deferred income taxes caused by the TCJA. The NOPR, therefore, no longer applies to DP&L. The rate changes will increase revenues by approximately $4.1 million through the end of 2020 as of the effective date, subject to refund based on final approved rates.

Regulatory Assets, Noncurrent

$ 0.8

Regulatory Assets, Noncurrent

$ 0.8

Increase in revenues from proposed transmission rate change to a formula rate

$ 4.1

Increase in revenues from proposed transmission rate change to a formula rate

$ 4.1