Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period		$ (3.6)			$ (3.6)
Other comprehensive loss before reclassifications					(0.1)
Amounts reclassified from AOCI to earnings					0.0
Other comprehensive income / (loss)	$ 0.2	(0.3)	$ (0.8)	$ (0.4)	(0.1)	$ (1.2)
Balance, end of period	(3.7)				(3.7)
Interest and Debt Expense	(19.4)		(21.7)		(38.3)	(45.4)
Income Tax Expense (Benefit)	(2.8)		3.6		(2.7)	2.7
Net income	3.5	1.9	(16.1)	42.1	5.4	26.0
Nonoperating Income (Expense)	(18.6)		(65.3)		(38.4)	(87.5)
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period		(36.9)			(36.9)
Other comprehensive loss before reclassifications					(0.1)
Amounts reclassified from AOCI to earnings					1.4
Other comprehensive income / (loss)	0.9	1.3	(0.9)	$ 0.4	1.3	(0.5)
Balance, end of period	(35.6)				(35.6)
Interest and Debt Expense	(6.4)		(6.7)		(12.6)	(13.8)
Income Tax Expense (Benefit)	2.3		(6.2)		1.1	(12.3)
Net income	17.8		29.8		29.5	58.8
Nonoperating Income (Expense)	(6.2)		(6.7)		(14.2)	(13.2)
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			14.5
Other comprehensive loss before reclassifications					(0.1)
Amounts reclassified from AOCI to earnings					(0.5)
Other comprehensive income / (loss)					(0.6)
Balance, end of period	13.9				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)			(0.4)
Other comprehensive loss before reclassifications					(0.1)
Amounts reclassified from AOCI to earnings					(0.2)
Other comprehensive income / (loss)					(0.3)
Balance, end of period	(0.7)				(0.7)
Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period		(18.1)			(18.1)
Other comprehensive loss before reclassifications					0.0
Amounts reclassified from AOCI to earnings					0.5
Other comprehensive income / (loss)					0.5
Balance, end of period	(17.6)				(17.6)
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)			(36.5)
Other comprehensive loss before reclassifications					0.0
Amounts reclassified from AOCI to earnings					1.6
Other comprehensive income / (loss)					1.6
Balance, end of period	(34.9)				(34.9)
Reclassification out of Accumulated Other Comprehensive Income [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net income	0.0		(0.3)		0.0	(0.4)
Reclassification out of Accumulated Other Comprehensive Income [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net income	0.7		(0.3)		1.4	0.3
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Interest and Debt Expense	(0.2)		(0.3)		(0.6)	(0.6)
Income Tax Expense (Benefit)	0.0		0.0		0.1	0.1
Net income	(0.2)		(0.3)		(0.5)	(0.5)
Reclassification out of Accumulated Other Comprehensive Income [Member] | 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]
Interest and Debt Expense	(0.1)		0.0		(0.2)	(0.1)
Income Tax Expense (Benefit)	0.0		0.0		0.0	0.0
Net income	(0.1)		0.0		(0.2)	(0.1)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Income Tax Expense (Benefit)	(0.1)		0.0		(0.1)	0.0
Net income	0.2		0.0		0.5	0.1
Nonoperating Income (Expense)	0.3		0.0		0.6	0.1
Reclassification out of Accumulated Other Comprehensive Income [Member] | Change in unfunded pension obligation [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Income Tax Expense (Benefit)	(0.2)		(1.2)		(0.4)	(1.4)
Net income	0.8		(0.3)		1.6	0.4
Nonoperating Income (Expense)	$ 1.0		$ 0.9		$ 2.0	$ 1.8