Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 7.5	$ 24.6
Other comprehensive loss before reclassifications	19.7	(34.2)
Amounts reclassified from accumulated other comprehensive income / (loss)	(9.8)	17.1
Other comprehensive income / (loss)	9.9	(17.1)	$ 28.5
Balance, end of period	17.4	7.5	24.6
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(42.3)	(26.7)
Other comprehensive loss before reclassifications	19.7	(33.9)
Amounts reclassified from accumulated other comprehensive income / (loss)	(6.1)	18.3
Other comprehensive income / (loss)	13.6	(15.6)	12.0
Balance, end of period	(28.7)	(42.3)	(26.7)
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	0.5	0.6
Other comprehensive loss before reclassifications	(0.1)	(0.3)
Amounts reclassified from accumulated other comprehensive income / (loss)	0.0	0.2
Other comprehensive income / (loss)	(0.1)	(0.1)
Balance, end of period	0.4	0.5	0.6
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	0.7	0.8
Other comprehensive loss before reclassifications	(0.2)	(0.3)
Amounts reclassified from accumulated other comprehensive income / (loss)	0.0	0.2
Other comprehensive income / (loss)	(0.2)	(0.1)
Balance, end of period	0.5	0.7	0.8
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	18.5	20.6
Other comprehensive loss before reclassifications	18.2	(19.0)
Amounts reclassified from accumulated other comprehensive income / (loss)	(10.0)	16.9
Other comprehensive income / (loss)	8.2	(2.1)
Balance, end of period	26.7	18.5	20.6
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	2.8	6.2
Other comprehensive loss before reclassifications	18.2	(18.8)
Amounts reclassified from accumulated other comprehensive income / (loss)	(9.8)	15.4
Other comprehensive income / (loss)	8.4	(3.4)
Balance, end of period	11.2	2.8	6.2
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(11.5)	3.4
Other comprehensive loss before reclassifications	1.6	(14.9)
Amounts reclassified from accumulated other comprehensive income / (loss)	0.2	0.0
Other comprehensive income / (loss)	1.8	(14.9)
Balance, end of period	(9.7)	(11.5)	3.4
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(45.8)	(33.7)
Other comprehensive loss before reclassifications	1.7	(14.8)
Amounts reclassified from accumulated other comprehensive income / (loss)	3.7	2.7
Other comprehensive income / (loss)	5.4	(12.1)
Balance, end of period	$ (40.4)	$ (45.8)	$ (33.7)