Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			$ 17.4
Other comprehensive income before reclassifications			22.6
Amounts reclassified from accumulated other comprehensive income / (loss)			(24.3)
Other comprehensive income / (loss)	$ 4.1	$ 5.6	(1.7)	$ 5.9
Balance, end of period	15.7		15.7
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(28.7)
Other comprehensive income before reclassifications			22.7
Amounts reclassified from accumulated other comprehensive income / (loss)			(23.1)
Other comprehensive income / (loss)	4.7	$ 6.3	(0.4)	$ 8.6
Balance, end of period	(29.1)		(29.1)
Gains / (losses) on available-for-sale securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			0.4
Other comprehensive income before reclassifications			0.2
Amounts reclassified from accumulated other comprehensive income / (loss)			0.0
Other comprehensive income / (loss)			0.2
Balance, end of period	0.6		0.6
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			0.5
Other comprehensive income before reclassifications			0.2
Amounts reclassified from accumulated other comprehensive income / (loss)			0.0
Other comprehensive income / (loss)			0.2
Balance, end of period	0.7		0.7
Gains / (losses) on cash flow hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			26.7
Other comprehensive income before reclassifications			22.4
Amounts reclassified from accumulated other comprehensive income / (loss)			(24.4)
Other comprehensive income / (loss)			(2.0)
Balance, end of period	24.7		24.7
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			11.2
Other comprehensive income before reclassifications			22.5
Amounts reclassified from accumulated other comprehensive income / (loss)			(24.8)
Other comprehensive income / (loss)			(2.3)
Balance, end of period	8.9		8.9
Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(9.7)
Other comprehensive income before reclassifications			0.0
Amounts reclassified from accumulated other comprehensive income / (loss)			0.1
Other comprehensive income / (loss)			0.1
Balance, end of period	(9.6)		(9.6)
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			(40.4)
Other comprehensive income before reclassifications			0.0
Amounts reclassified from accumulated other comprehensive income / (loss)			1.7
Other comprehensive income / (loss)			1.7
Balance, end of period	$ (38.7)		$ (38.7)