Supplemental Financial Information (Reclassification out of ACOI) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other deductions	$ (113.9)	$ (109.4)	$ (121.5)
Interest expense	(110.1)	(107.7)	(119.8)
Revenue	1,236.9	1,427.3	1,612.8
Purchased Power	(339.2)	(417.4)	(562.6)
Tax expense (benefit)	25.3	278.8	(20.0)
Income / (loss) from discontinued operations	0.0	(0.7)	11.4
Income tax expense / (benefit) from discontinued operations	0.0	19.2	(1.0)
Net income (loss)	(94.6)	(485.2)	(239.0)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other deductions	(32.1)	(24.5)	(33.2)
Interest expense	(30.5)	(24.7)	(28.9)
Revenue	720.0	808.0	857.0
Purchased Power	(289.8)	(316.7)	(317.4)
Tax expense (benefit)	(31.1)	(46.0)	(59.0)
Income / (loss) from discontinued operations	(56.3)	(1,338.7)	(47.5)
Income tax expense / (benefit) from discontinued operations	(15.9)	(468.4)	(23.9)
Net income (loss)	17.0	(772.7)	106.4
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income (loss)	(7.1)	(28.7)	(9.8)
Reclassification out of Accumulated Other Comprehensive Income [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income (loss)	(1.8)	(24.1)	(6.1)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other deductions	(0.1)	0.0	0.0
Tax expense (benefit)	0.0	0.0	0.0
Net income (loss)	(0.1)	0.0	0.0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other deductions	(0.1)	0.0	0.0
Tax expense (benefit)	0.0	0.0	0.0
Net income (loss)	(0.1)	0.0	0.0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Interest expense	(1.0)	(1.0)	(1.1)
Revenue	(15.2)	(55.3)	(18.7)
Purchased Power	3.8	9.9	4.4
Total before income taxes	(12.4)	(46.4)	(15.4)
Tax expense (benefit)	4.4	16.7	5.4
Net income (loss)	(8.0)	(29.7)	(10.0)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Interest expense	(0.9)	(1.0)	(1.1)
Tax expense (benefit)	0.2	0.2	0.2
Income / (loss) from discontinued operations	(8.5)	(45.4)	(14.3)
Income tax expense / (benefit) from discontinued operations	(3.0)	(16.2)	(5.4)
Net income (loss)	(6.2)	(30.0)	(9.8)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other deductions	1.5	1.6	0.4
Tax expense (benefit)	(0.5)	(0.6)	(0.2)
Net income (loss)	1.0	1.0	0.2
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other deductions	6.8	7.7	5.6
Tax expense (benefit)	(2.3)	(1.8)	(1.9)
Net income (loss)	$ 4.5	$ 5.9	$ 3.7