Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Supplemental Financial Information [Line Items]
Other comprehensive loss before reclassifications	$ (0.3)
Allowance for Doubtful Accounts Receivable, Current	0.7		$ 0.4
Customer receivables	45.7		46.3
Unbilled Revenue	15.9		19.4
Amounts due from partners in jointly owned stations	0.2		0.3
Due from PJM transmission settlement	1.8		1.8
Other	2.0		1.2
Provision for uncollectible accounts	(0.7)		(0.4)
Total accounts receivable, net	64.9		68.6
Fuel and limestone	1.3		3.5
Plant materials and supplies	9.4		10.6
Total inventories, at average cost	10.7		14.1
Other Operating Income (Expense), Net	0.1	$ (0.9)
Provision for Doubtful Accounts	0.4
Allowance for Doubtful Accounts Receivable, Write-offs	(0.1)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0
Other Comprehensive Income (Loss), Net of Tax	(0.3)	(0.4)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Other comprehensive loss before reclassifications	(0.3)
Allowance for Doubtful Accounts Receivable, Current	0.7		0.4
Customer receivables	44.5		45.0
Unbilled Revenue	15.9		19.4
Amounts due from partners in jointly owned stations	4.6		3.9
Due from PJM transmission settlement	1.8		1.8
Other	2.1		1.2
Provision for uncollectible accounts	(0.7)		(0.4)
Total accounts receivable, net	68.2		70.9
Total inventories, at average cost	9.1		$ 10.4
Provision for Doubtful Accounts	0.4
Allowance for Doubtful Accounts Receivable, Write-offs	(0.1)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.7)
Other Comprehensive Income (Loss), Net of Tax	0.4	$ 0.4
Amortization of defined benefit pension items [Member]
Supplemental Financial Information [Line Items]
Other comprehensive loss before reclassifications	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.3)
Other Comprehensive Income (Loss), Net of Tax	0.3
Amortization of defined benefit pension items [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Other comprehensive loss before reclassifications	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.8)
Other Comprehensive Income (Loss), Net of Tax	0.8
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Supplemental Financial Information [Line Items]
Other comprehensive loss before reclassifications	(0.3)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.3
Other Comprehensive Income (Loss), Net of Tax	(0.6)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Other comprehensive loss before reclassifications	(0.3)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.1
Other Comprehensive Income (Loss), Net of Tax	$ (0.4)