Assets and Liabilities Held-For-Sale and Dispositions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	$ 31.9	$ (6.5)	$ 12.4
Accounts receivable, net	4.0	37.9
Fixed-asset impairment (Note 14)	2.8	0.0	23.9
Disposal Group, Including Discontinued Operation, Inventory	0.0	19.4
Disposal Group, Including Discontinued Operations, Taxes Applicable to Subsequent Years	2.3	7.4
Property, plant & equipment, net	0.0	232.2
Total assets of the disposal group classified as held for sale in the balance sheets	14.0	321.9
Accounts payable	3.9	25.1
Disposal Group, Including Discontinued Operation, Accrued Income Tax Payable	3.1	6.3
Disposal Group, Including Discontinued Operation, Taxes Payable	2.3	7.4
Asset Retirement Obligation, Held for Sale	90.4	116.6
Total liabilities of the disposal group classified as held for sale in the balance sheets	81.4	199.9
Proceeds from disposal and sale of business	234.9	70.1	0.0
Gain (Loss) on Disposition of Business	(11.7)	0.0	0.0
Property, Plant and Equipment, Additions	103.6	121.5	148.5
Beckjord [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(11.7)
Property, Plant and Equipment, Additions	14.5
THE DAYTON POWER AND LIGHT COMPANY [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	104.4	88.5	143.6
Fixed-asset impairment (Note 14)	0.0	66.3	1,353.5
Gain (Loss) on Disposition of Business	(12.4)	$ 0.0	$ 0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Beckjord [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(12.4)
Property, Plant and Equipment, Additions	$ 14.5