Supplemental Financial Information (Reclassification out of ACOI) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income	$ (22.3)	$ (28.4)
Tax expense	(2.7)	(0.2)
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	29.8	20.0
Discontinued Operation, Tax Effect of Discontinued Operation	(4.3)	(3.5)
Net income / (loss)	42.1	16.9
Interest expense	(23.7)	(27.8)
Operating Expenses	(89.8)	(86.0)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income	(6.5)	(9.5)
Tax expense	(6.1)	(3.6)
Net income / (loss)	29.0	15.7
Interest expense	(7.1)	(8.2)
Operating Expenses	(85.6)	(82.5)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income / (loss)	(0.1)	2.4
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)	0.6	0.6
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]
Tax expense	0.1	0.6
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	0.0	4.1
Discontinued Operation, Tax Effect of Discontinued Operation	0.0	(1.4)
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	0.0	2.7
Net income / (loss)	(0.2)	(0.4)
Interest expense	(0.3)	(1.0)
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]
Tax expense	0.0	0.5
Net income / (loss)	(0.1)	(0.3)
Interest expense	(0.1)	(0.8)
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]
Other income	0.1	0.2
Tax expense	0.0	(0.1)
Net income / (loss)	0.1	0.1
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]
Other income	0.9	1.1
Tax expense	(0.2)	(0.2)
Net income / (loss)	$ 0.7	$ 0.9