Equity (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 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)	$ (2,740.0)	$ (2,739.0)
Common stock, shares authorized | shares	1,500	1,500
Common stock, shares outstanding | shares	1	1
Accounts Payable, Related Parties, Current	$ 14.8	$ 11.0
Non-cash capital contribution			$ 40.0
Proceeds from Contributions from Parent	(98.0)	0.0	0.0
Cash Contribution from Parent Company	150.0
Payment against tax receivable balance from Parent	52.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Class of Stock [Line Items]
Retained earnings / (deficit)	$ (56.0)	$ (107.1)
Common stock, shares authorized | shares	50,000,000
Common stock, shares outstanding | shares	41,172,173	41,172,173
Accounts Payable, Related Parties, Current	$ 14.8	$ 11.0
Proceeds from Contributions from Parent	(150.0)	0.0	(80.0)
Dividends, Common Stock, Cash	52.7	(95.0)	43.8
Payments of Ordinary Dividends, Common Stock	42.7	43.8	43.8
Other Paid-In Capital [Member]
Class of Stock [Line Items]
Non-cash capital contribution	0.0	0.0	40.0
Proceeds from Contributions from Parent	(98.0)
Other Paid-In Capital [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Class of Stock [Line Items]
Proceeds from Contributions from Parent	150.0		80.0
Dividends, Common Stock, Cash	52.7	95.0	43.8
Payments of Ordinary Dividends, Common Stock		95.0
Dividends, Common Stock	42.7
Generation [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Class of Stock [Line Items]
Disposal Group, Including Discontinued Operation, Net Assets	$ 0.0	$ 0.0	$ 10.0