Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 2.2
Other comprehensive income before reclassifications	(0.3)
Amounts reclassified from AOCI to earnings	(0.1)
Other comprehensive income / (loss)	(0.4)
Balance, end of period	1.8
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(35.3)
Other comprehensive income before reclassifications	(0.2)
Amounts reclassified from AOCI to earnings	0.6
Other comprehensive income / (loss)	0.4	$ 1.1
Balance, end of period	(34.9)
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 income before reclassifications	(0.3)
Amounts reclassified from AOCI to earnings	(0.2)
Other comprehensive income / (loss)	(0.5)
Balance, end of period	16.5
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 income before reclassifications	(0.2)
Amounts reclassified from AOCI to earnings	(0.1)
Other comprehensive income / (loss)	(0.3)
Balance, end of period	0.3
Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(14.8)
Other comprehensive income before reclassifications	0.0
Amounts reclassified from AOCI to earnings	0.1
Other comprehensive income / (loss)	0.1
Balance, end of period	(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 income before reclassifications	0.0
Amounts reclassified from AOCI to earnings	0.7
Other comprehensive income / (loss)	0.7
Balance, end of period	$ (35.2)