Generation Separation (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 01, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long Term Indebtedness, Less than or Equal to	$ 750,000,000.0
Cash Provided by (Used in) Operating Activities, Discontinued Operations			$ (700,000)	$ 35,800,000
Cash Provided by (Used in) Investing Activities, Discontinued Operations			75,500,000	500,000
Revenues			0	340,900,000
Cost of revenues			0	(307,000,000)
Income / (loss) from discontinued operations		$ 0	(700,000)	11,400,000
Income tax expense / (benefit) from discontinued operations		0	19,200,000	(1,000,000)
Net income from discontinued operations		0	29,300,000	12,400,000
THE DAYTON POWER AND LIGHT COMPANY [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long Term Indebtedness, Less than or Equal to	$ 750,000,000.0	$ 750,000,000
Debt Percentage of Rate Base		75.00%
Income / (loss) from discontinued operations		$ (56,300,000)	(1,338,700,000)	(47,500,000)
Income tax expense / (benefit) from discontinued operations		(15,900,000)	(468,400,000)	(23,900,000)
Net income from discontinued operations		(40,400,000)	(870,300,000)	(23,600,000)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Transmission and Distribution [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Debt, Gross					$ 750,000,000
THE DAYTON POWER AND LIGHT COMPANY [Member] | Generation [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash Provided by (Used in) Operating Activities, Discontinued Operations		(10,400,000)	50,900,000	138,700,000
Cash Provided by (Used in) Investing Activities, Discontinued Operations		23,400,000	(50,900,000)	(24,300,000)
Cash Provided by (Used in) Financing Activities, Discontinued Operations		(13,000,000)	0	(114,400,000)
Restricted Cash			29,000,000		2,000,000
Accounts receivable, net			34,900,000		31,300,000
Inventory, Net			66,500,000		42,000,000
Disposal Group, Including Discontinued Operations, Taxes Applicable to Subsequent Years			11,300,000		1,800,000
Property, plant & equipment, net			156,700,000		87,000,000
Intangible assets, net			900,000		700,000
Other assets			25,300,000		15,500,000
Total assets of the disposal group classified as held for sale in the balance sheets			324,600,000		180,300,000
Accounts payable			54,800,000		12,400,000
Disposal Group, Including Discontinued Operation, Accrued Income Tax Payable			3,500,000		(3,900,000)
Disposal Group, Including Discontinued Operations, Long-Term Debt			13,400,000		300,000
Disposal Group, Including Discontinued Operation, Taxes Payable			11,300,000
Disposal Group, Including Discontinued Operation, Deferred Tax Liabilities			(120,700,000)		(91,900,000)
Disposal Group, Including Discontinued Operation, Pension Plan Benefit Obligation			8,200,000		9,600,000
Disposal Group, Including Discontinued Operation, Accumulated Deferred Investment Tax Credit			16,600,000		15,100,000
Asset Retirement Obligation, Held for Sale			127,000,000		126,300,000
Other liabilities			43,600,000		24,100,000
Total liabilities of the disposal group classified as held for sale in the balance sheets			157,700,000		92,000,000
Disposal Group, Including Discontinued Operation, Accumulated Other Comprehensive Income (Loss), Net of Tax					2,100,000
Disposal Group, Including Discontinued Operation, Net Assets					$ 86,200,000
Revenues		358,400,000	557,900,000	901,600,000
Cost of revenues		(191,600,000)	(341,100,000)	(698,300,000)
Disposal Group, Including Discontinued Operation, Operating and Other Expenses		(156,800,000)	(202,000,000)	(250,800,000)
Disposal Group, Including Discontinued Operation, Fixed-Asset Impairment		(66,300,000)	(1,353,500,000)	0
Income / (loss) from discontinued operations		(56,300,000)	(1,338,700,000)	(47,500,000)
Income tax expense / (benefit) from discontinued operations		(15,900,000)	(468,400,000)	(23,900,000)
Net income from discontinued operations		(40,400,000)	(870,300,000)	(23,600,000)
Disposal Group, Including Discontinued Operation, Interest Expense		$ 200,000	$ 500,000	$ 2,900,000