Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 0.3
Other comprehensive income before reclassifications	3.9
Amounts reclassified from accumulated other comprehensive income / (loss)	1.7
Other comprehensive income / (loss)	5.6	$ 13.4
Balance, end of period	5.9
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(42.5)
Other comprehensive income before reclassifications	3.8
Amounts reclassified from accumulated other comprehensive income / (loss)	3.4
Other comprehensive income / (loss)	7.2	$ 13.5
Balance, end of period	(35.3)
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 income before reclassifications	0.2
Amounts reclassified from accumulated other comprehensive income / (loss)	(0.1)
Other comprehensive income / (loss)	0.1
Balance, end of period	0.7
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	0.2
Amounts reclassified from accumulated other comprehensive income / (loss)	(0.1)
Other comprehensive income / (loss)	0.1
Balance, end of period	0.8
Gains / (losses) on cash flow hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	13.1
Other comprehensive income before reclassifications	5.2
Amounts reclassified from accumulated other comprehensive income / (loss)	1.0
Other comprehensive income / (loss)	6.2
Balance, end of period	19.3
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 income before reclassifications	5.2
Amounts reclassified from accumulated other comprehensive income / (loss)	1.0
Other comprehensive income / (loss)	6.2
Balance, end of period	3.5
Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(13.4)
Other comprehensive income before reclassifications	(1.5)
Amounts reclassified from accumulated other comprehensive income / (loss)	0.8
Other comprehensive income / (loss)	(0.7)
Balance, end of period	(14.1)
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 income before reclassifications	(1.6)
Amounts reclassified from accumulated other comprehensive income / (loss)	2.5
Other comprehensive income / (loss)	0.9
Balance, end of period	$ (39.6)