Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			$ 7.5
Other comprehensive income before reclassifications			9.4
Amounts reclassified from accumulated other comprehensive income / (loss)			(3.5)
Other comprehensive income / (loss)	$ 5.6	$ 4.4	5.9	$ (9.8)
Balance, end of period	13.4		13.4
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(42.3)
Other comprehensive income before reclassifications			9.3
Amounts reclassified from accumulated other comprehensive income / (loss)			(0.7)
Other comprehensive income / (loss)	6.3	$ 5.1	8.6	$ (9.1)
Balance, end of period	(33.7)		(33.7)
Gains / (losses) on available-for-sale securities [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	0.3		0.3
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 income before reclassifications			(0.3)
Amounts reclassified from accumulated other comprehensive income / (loss)			0.0
Other comprehensive income / (loss)			(0.3)
Balance, end of period	0.4		0.4
Gains / (losses) on cash flow hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			18.5
Other comprehensive income before reclassifications			9.6
Amounts reclassified from accumulated other comprehensive income / (loss)			(3.4)
Other comprehensive income / (loss)			6.2
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			2.8
Other comprehensive income before reclassifications			9.6
Amounts reclassified from accumulated other comprehensive income / (loss)			(3.3)
Other comprehensive income / (loss)			6.3
Balance, end of period	9.1		9.1
Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(11.5)
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	(11.6)		(11.6)
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			(45.8)
Other comprehensive income before reclassifications			0.0
Amounts reclassified from accumulated other comprehensive income / (loss)			2.6
Other comprehensive income / (loss)			2.6
Balance, end of period	$ (43.2)		$ (43.2)