Equity (Narrative) (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Oct. 01, 2017 USD ($)	Jan. 01, 2016 USD ($)
Class of Stock [Line Items]
Maximum leverage ratio to allow distribution to shareholder	0.67
Minimum coverage ratio to allow distribution to shareholder	2.50
Retained earnings / (deficit)	$ (2,915.5)	$ (2,820.9)
Common stock, shares authorized | shares	1,500	1,500
Common stock, shares outstanding | shares	1	1
Leverage Ratio	1.50
Accounts Payable, Related Parties, Current	$ 3.9	$ 2.0
PUCO Equity Ratio	33.00%
Non-cash capital contribution	$ 97.1	0.0	$ 0.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Class of Stock [Line Items]
Retained earnings / (deficit)	$ (319.3)	$ (406.3)
Common stock, shares authorized | shares	250,000,000	250,000,000
Common stock, shares outstanding | shares	41,172,173	41,172,173
PUCO Meger Equity Ratio Approval	50.00%
Accounts Payable, Related Parties, Current	$ 3.9	$ 2.0
PUCO Equity Ratio	33.00%
Proceeds from Contributions from Parent	$ 70.0	0.0	0.0
Dividends, Common Stock, Cash	39.0	(70.0)	50.0
Payments of Ordinary Dividends, Common Stock	$ 39.0	70.0	50.0
DP&L Series A [Member]
Class of Stock [Line Items]
Temporary Equity, Preferred Stock Rate	3.75%
Temporary Equity, Redemption Price Per Share | $ / shares	$ 102.50
DP&L Series A [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Class of Stock [Line Items]
Temporary Equity, Preferred Stock Rate	3.75%
Temporary Equity, Redemption Price Per Share | $ / shares	$ 102.50
DP&L Series B [Member]
Class of Stock [Line Items]
Temporary Equity, Preferred Stock Rate	3.75%
Temporary Equity, Redemption Price Per Share | $ / shares	$ 103.00
DP&L Series B [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Class of Stock [Line Items]
Temporary Equity, Preferred Stock Rate	3.75%
Temporary Equity, Redemption Price Per Share | $ / shares	$ 103.00
DP&L Series C [Member]
Class of Stock [Line Items]
Temporary Equity, Preferred Stock Rate	3.90%
Temporary Equity, Redemption Price Per Share | $ / shares	$ 101.00
DP&L Series C [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Class of Stock [Line Items]
Temporary Equity, Preferred Stock Rate	3.90%
Temporary Equity, Redemption Price Per Share | $ / shares	$ 101.00
Subsidiary of Common Parent [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Class of Stock [Line Items]
Accounts Payable, Related Parties, Current					$ 7.5
Equity Settlement of Related Party Payable	$ 0.0	$ 7.5	$ 0.0
Other Paid-In Capital [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Class of Stock [Line Items]
Proceeds from Contributions from Parent	(70.0)
Dividends, Common Stock, Cash	$ 39.0
Generation [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Class of Stock [Line Items]
Disposal Group, Including Discontinued Operation, Net Assets				$ 86.2