Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 0.3	$ 17.4
Other comprehensive income / (loss) before reclassifications	7.6	11.6
Amounts reclassified from accumulated other comprehensive income / (loss)	(7.1)	(28.7)
Other comprehensive income / (loss)	0.5	(17.1)	$ 9.9
Balance, end of period	0.8	0.3	17.4
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(42.5)	(28.7)
Other comprehensive income / (loss) before reclassifications	10.2	10.3
Stockholders' Equity Note, Spinoff Transaction	(88.3)
Amounts reclassified from accumulated other comprehensive income / (loss)	(1.8)	(24.1)
Other comprehensive income / (loss)	8.4	(13.8)	13.6
Balance, end of period	(36.2)	(42.5)	(28.7)
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	0.6	0.4
Other comprehensive income / (loss) before reclassifications	0.5	0.2
Amounts reclassified from accumulated other comprehensive income / (loss)	(0.1)	0.0
Other comprehensive income / (loss)	0.4	0.2
Balance, end of period	1.0	0.6	0.4
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	0.5
Other comprehensive income / (loss) before reclassifications	0.5	0.2
Amounts reclassified from accumulated other comprehensive income / (loss)	(0.1)	0.0
Other comprehensive income / (loss)	0.4	0.2
Balance, end of period	1.1	0.7	0.5
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	13.1	26.7
Other comprehensive income / (loss) before reclassifications	9.6	16.1
Amounts reclassified from accumulated other comprehensive income / (loss)	(8.0)	(29.7)
Other comprehensive income / (loss)	1.6	(13.6)
Balance, end of period	14.7	13.1	26.7
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	(2.7)	11.2
Other comprehensive income / (loss) before reclassifications	12.4	16.1
Stockholders' Equity Note, Spinoff Transaction	(2.1)
Amounts reclassified from accumulated other comprehensive income / (loss)	(6.2)	(30.0)
Other comprehensive income / (loss)	6.2	(13.9)
Balance, end of period	1.4	(2.7)	11.2
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(13.4)	(9.7)
Other comprehensive income / (loss) before reclassifications	(2.5)	(4.7)
Amounts reclassified from accumulated other comprehensive income / (loss)	1.0	1.0
Other comprehensive income / (loss)	(1.5)	(3.7)
Balance, end of period	(14.9)	(13.4)	(9.7)
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	(40.5)	(40.4)
Other comprehensive income / (loss) before reclassifications	(2.7)	(6.0)
Amounts reclassified from accumulated other comprehensive income / (loss)	4.5	5.9
Other comprehensive income / (loss)	1.8	(0.1)
Balance, end of period	$ (38.7)	$ (40.5)	$ (40.4)