Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period				$ 2.2
Other comprehensive loss before reclassifications				(1.0)
Amounts reclassified from AOCI to earnings				(1.2)
Other comprehensive income / (loss)	$ (1.0)	$ 0.0		(2.2)	$ 3.0
Balance, end of period	0.0			0.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period				(35.3)
Other comprehensive loss before reclassifications				(0.9)
Amounts reclassified from AOCI to earnings				2.0
Other comprehensive income / (loss)	1.6	$ 0.6	$ 0.8	1.1	$ 2.5
Balance, end of period	(34.2)			(34.2)
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				17.0
Other comprehensive loss before reclassifications				(1.0)
Amounts reclassified from AOCI to earnings				(1.3)
Other comprehensive income / (loss)				(2.3)
Balance, end of period	14.7			14.7
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				0.6
Other comprehensive loss before reclassifications				(0.9)
Amounts reclassified from AOCI to earnings				(0.1)
Other comprehensive income / (loss)				(1.0)
Balance, end of period	(0.4)			(0.4)
Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period				(14.8)
Other comprehensive loss before reclassifications				0.0
Amounts reclassified from AOCI to earnings				0.1
Other comprehensive income / (loss)				0.1
Balance, end of period	(14.7)			(14.7)
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				(35.9)
Other comprehensive loss before reclassifications				0.0
Amounts reclassified from AOCI to earnings				2.1
Other comprehensive income / (loss)				2.1
Balance, end of period	$ (33.8)			$ (33.8)