Supplemental Financial Information (Reclassification out of ACOI) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Interest expense	$ (18.3)	$ (22.6)	$ (63.7)	$ (74.4)
Tax expense	9.4	(3.1)	12.6	(1.5)
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(0.4)	5.0	32.8	36.7
Discontinued Operation, Tax Effect of Discontinued Operation	(0.1)	(1.0)	(7.1)	(5.9)
Nonoperating Income (Expense)	(18.0)	(22.1)	(105.4)	(80.0)
Net income	35.4	21.0	61.4	41.4
THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Interest expense	(6.1)	(5.8)	(19.9)	(20.5)
Tax expense	7.2	(6.3)	(5.1)	(12.0)
Nonoperating Income (Expense)	(6.6)	(6.2)	(19.8)	(23.0)
Net income	44.9	31.2	103.7	61.9
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income	(0.8)	(0.1)	(1.2)	2.6
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	1.7	0.6	2.0	2.0
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Interest expense	(0.3)	(0.3)	(0.9)	(0.9)
Tax expense	(0.1)	0.1	0.0	0.3
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	0.0	0.0	0.0	4.3
Discontinued Operation, Tax Effect of Discontinued Operation	(0.4)	0.0	(0.4)	(1.5)
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	(0.4)	0.0	(0.4)	2.8
Net income	(0.4)	(0.2)	(0.9)	(0.6)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | 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]
Interest expense	0.0	(0.3)	(0.1)	(1.1)
Tax expense	0.0	0.1	0.0	0.6
Net income	0.0	(0.2)	(0.1)	(0.5)
Amortization of defined benefit pension items [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Tax expense	(0.1)	(0.1)	(0.1)	(0.1)
Nonoperating Income (Expense)	0.1	0.2	0.2	0.5
Net income	0.0	0.1	0.1	0.4
Amortization of defined benefit pension items [Member] | 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]
Tax expense	0.9	(0.2)	(0.5)	(0.7)
Nonoperating Income (Expense)	0.8	1.0	2.6	3.2
Net income	$ 1.7	$ 0.8	$ 2.1	$ 2.5