Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 17.4
Other comprehensive income before reclassifications	21.7
Amounts reclassified from accumulated other comprehensive income / (loss)	(8.3)
Other comprehensive income	13.4	$ 0.9
Balance, end of period	30.8
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	21.7
Amounts reclassified from accumulated other comprehensive income / (loss)	(8.2)
Other comprehensive income	13.5	$ 1.6
Balance, end of period	(15.2)
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)	(0.1)
Other comprehensive income	0.1
Balance, end of period	0.5
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)	(0.1)
Other comprehensive income	0.1
Balance, end of period	0.6
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	21.5
Amounts reclassified from accumulated other comprehensive income / (loss)	(8.2)
Other comprehensive income	13.3
Balance, end of period	40.0
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	21.5
Amounts reclassified from accumulated other comprehensive income / (loss)	(8.4)
Other comprehensive income	13.1
Balance, end of period	24.3
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)	0.0
Other comprehensive income	0.0
Balance, end of period	(9.7)
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	(40.4)
Other comprehensive income before reclassifications	0.0
Amounts reclassified from accumulated other comprehensive income / (loss)	0.3
Other comprehensive income	0.3
Balance, end of period	$ (40.1)