Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			$ 0.3
Other comprehensive loss before reclassifications			4.9
Amounts reclassified from accumulated other comprehensive income / (loss)			1.8
Other comprehensive loss	$ 1.1	$ (19.2)	6.7	$ (5.9)
Balance, end of period	7.0		7.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(42.5)
Other comprehensive loss before reclassifications			4.9
Amounts reclassified from accumulated other comprehensive income / (loss)			4.0
Other comprehensive loss	1.7	$ (18.6)	8.9	$ (5.1)
Balance, end of period	(33.6)		(33.6)
Gains / (losses) on available-for-sale securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			0.6
Other comprehensive loss before reclassifications			0.3
Amounts reclassified from accumulated other comprehensive income / (loss)			(0.1)
Other comprehensive loss			0.2
Balance, end of period	0.8		0.8
Gains / (losses) on available-for-sale securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			0.7
Other comprehensive loss before reclassifications			0.3
Amounts reclassified from accumulated other comprehensive income / (loss)			(0.1)
Other comprehensive loss			0.2
Balance, end of period	0.9		0.9
Gains / (losses) on cash flow hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			13.1
Other comprehensive loss before reclassifications			6.1
Amounts reclassified from accumulated other comprehensive income / (loss)			1.0
Other comprehensive loss			7.1
Balance, end of period	20.2		20.2
Gains / (losses) on cash flow hedges [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(2.7)
Other comprehensive loss before reclassifications			6.2
Amounts reclassified from accumulated other comprehensive income / (loss)			0.9
Other comprehensive loss			7.1
Balance, end of period	4.4		4.4
Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(13.4)
Other comprehensive loss before reclassifications			(1.5)
Amounts reclassified from accumulated other comprehensive income / (loss)			0.9
Other comprehensive loss			(0.6)
Balance, end of period	(14.0)		(14.0)
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			(40.5)
Other comprehensive loss before reclassifications			(1.6)
Amounts reclassified from accumulated other comprehensive income / (loss)			3.2
Other comprehensive loss			1.6
Balance, end of period	$ (38.9)		$ (38.9)