Income Taxes (Reconciliation of Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 3.7	$ 3.0
Tax positions taken during prior periods	0.0	2.2
Lapse of applicable statute of limitations	(0.2)	(1.5)
Balance at end of year	3.5	3.7
THE DAYTON POWER AND LIGHT COMPANY [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	4.9	3.0
Tax positions taken during prior periods	0.0	3.4
Lapse of applicable statute of limitations	(0.1)	(1.5)
Balance at end of year	$ 4.8	$ 4.9