Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ (3.6)
Other comprehensive loss before reclassifications	(0.3)
Amounts reclassified from AOCI to earnings	0.0
Other comprehensive loss	(0.3)	$ (0.4)
Balance, end of period	(3.9)
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(36.9)
Other comprehensive loss before reclassifications	(0.3)
Amounts reclassified from AOCI to earnings	0.7
Other comprehensive loss	0.4	$ 0.4
Balance, end of period	(36.5)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	14.5
Other comprehensive loss before reclassifications	(0.3)
Amounts reclassified from AOCI to earnings	(0.3)
Other comprehensive loss	(0.6)
Balance, end of period	13.9
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(0.4)
Other comprehensive loss before reclassifications	(0.3)
Amounts reclassified from AOCI to earnings	(0.1)
Other comprehensive loss	(0.4)
Balance, end of period	(0.8)
Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(18.1)
Other comprehensive loss before reclassifications	0.0
Amounts reclassified from AOCI to earnings	0.3
Other comprehensive loss	0.3
Balance, end of period	(17.8)
Change in unfunded pension obligation [Member] \| THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(36.5)
Other comprehensive loss before reclassifications	0.0
Amounts reclassified from AOCI to earnings	0.8
Other comprehensive loss	0.8
Balance, end of period	$ (35.7)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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