Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			$ 7.5
Other comprehensive income before reclassifications			1.9
Amounts reclassified from accumulated other comprehensive income / (loss)			(1.6)
Other comprehensive income / (loss)	$ (0.6)	$ (6.7)	0.3	$ (14.2)
Balance, end of period	7.8		7.8
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			1.8
Amounts reclassified from accumulated other comprehensive income / (loss)			0.5
Other comprehensive income / (loss)	0.7	$ (7.5)	2.3	$ (14.2)
Balance, end of period	(40.0)		(40.0)
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			0.1
Amounts reclassified from accumulated other comprehensive income / (loss)			0.0
Other comprehensive income / (loss)			0.1
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			0.7
Other comprehensive income before reclassifications			0.0
Amounts reclassified from accumulated other comprehensive income / (loss)			0.0
Other comprehensive income / (loss)			0.0
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			18.5
Other comprehensive income before reclassifications			1.8
Amounts reclassified from accumulated other comprehensive income / (loss)			(1.4)
Other comprehensive income / (loss)			0.4
Balance, end of period	18.9		18.9
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			1.8
Amounts reclassified from accumulated other comprehensive income / (loss)			(1.3)
Other comprehensive income / (loss)			0.5
Balance, end of period	3.3		3.3
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)			(0.2)
Other comprehensive income / (loss)			(0.2)
Balance, end of period	(11.7)		(11.7)
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			(45.8)
Other comprehensive income before reclassifications			0.0
Amounts reclassified from accumulated other comprehensive income / (loss)			1.8
Other comprehensive income / (loss)			1.8
Balance, end of period	$ (44.0)		$ (44.0)