Supplemental Financial Information (Reclassification out of ACOI) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Interest expense	$ (19.4)		$ (21.7)		$ (38.3)	$ (45.4)
Tax expense	(2.8)		3.6		(2.7)	2.7
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax	(1.2)		2.8		(2.0)	31.1
Discontinued Operation, Tax Effect of Discontinued Operation	(0.7)		(0.4)		(0.5)	(6.5)
Nonoperating Income (Expense)	(18.6)		(65.3)		(38.4)	(87.5)
Net income / (loss)	3.5	$ 1.9	(16.1)	$ 42.1	5.4	26.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Interest expense	(6.4)		(6.7)		(12.6)	(13.8)
Tax expense	2.3		(6.2)		1.1	(12.3)
Nonoperating Income (Expense)	(6.2)		(6.7)		(14.2)	(13.2)
Net income / (loss)	17.8		29.8		29.5	58.8
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income / (loss)	0.0		(0.3)		0.0	(0.4)
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)	0.7		(0.3)		1.4	0.3
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	(0.2)		(0.3)		(0.6)	(0.6)
Tax expense	0.0		0.0		0.1	0.1
Net income / (loss)	(0.2)		(0.3)		(0.5)	(0.5)
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	(0.1)		0.0		(0.2)	(0.1)
Tax expense	0.0		0.0		0.0	0.0
Net income / (loss)	(0.1)		0.0		(0.2)	(0.1)
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)		0.0		(0.1)	0.0
Nonoperating Income (Expense)	0.3		0.0		0.6	0.1
Net income / (loss)	0.2		0.0		0.5	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]
Tax expense	(0.2)		(1.2)		(0.4)	(1.4)
Nonoperating Income (Expense)	1.0		0.9		2.0	1.8
Net income / (loss)	$ 0.8		$ (0.3)		$ 1.6	$ 0.4