Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 17.4	$ 7.5
Other comprehensive income / (loss) before reclassifications	11.6	19.7
Amounts reclassified from accumulated other comprehensive income / (loss)	(28.7)	(9.8)
Other comprehensive income / (loss)	(17.1)	9.9	$ (17.1)
Balance, end of period	0.3	17.4	7.5
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(28.7)	(42.3)
Other comprehensive income / (loss) before reclassifications	10.3	19.7
Amounts reclassified from accumulated other comprehensive income / (loss)	(24.1)	(6.1)
Other comprehensive income / (loss)	(13.8)	13.6	(15.6)
Balance, end of period	(42.5)	(28.7)	(42.3)
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	0.4	0.5
Other comprehensive income / (loss) before reclassifications	0.2	(0.1)
Amounts reclassified from accumulated other comprehensive income / (loss)	0.0	0.0
Other comprehensive income / (loss)	0.2	(0.1)
Balance, end of period	0.6	0.4	0.5
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	0.5	0.7
Other comprehensive income / (loss) before reclassifications	0.2	(0.2)
Amounts reclassified from accumulated other comprehensive income / (loss)	0.0	0.0
Other comprehensive income / (loss)	0.2	(0.2)
Balance, end of period	0.7	0.5	0.7
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	26.7	18.5
Other comprehensive income / (loss) before reclassifications	16.1	18.2
Amounts reclassified from accumulated other comprehensive income / (loss)	(29.7)	(10.0)
Other comprehensive income / (loss)	(13.6)	8.2
Balance, end of period	13.1	26.7	18.5
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	11.2	2.8
Other comprehensive income / (loss) before reclassifications	16.1	18.2
Amounts reclassified from accumulated other comprehensive income / (loss)	(30.0)	(9.8)
Other comprehensive income / (loss)	(13.9)	8.4
Balance, end of period	(2.7)	11.2	2.8
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(9.7)	(11.5)
Other comprehensive income / (loss) before reclassifications	(4.7)	1.6
Amounts reclassified from accumulated other comprehensive income / (loss)	1.0	0.2
Other comprehensive income / (loss)	(3.7)	1.8
Balance, end of period	(13.4)	(9.7)	(11.5)
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	(40.4)	(45.8)
Other comprehensive income / (loss) before reclassifications	(6.0)	1.7
Amounts reclassified from accumulated other comprehensive income / (loss)	5.9	3.7
Other comprehensive income / (loss)	(0.1)	5.4
Balance, end of period	$ (40.5)	$ (40.4)	$ (45.8)