Income Taxes (Reconciliation of Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 3.0	$ 3.0
Tax positions taken during prior periods	2.2	0.0
Lapse of applicable statute of limitations	(1.5)	0.0
Balance at end of year	3.7	3.0
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	3.0	3.0
Tax positions taken during prior periods	3.4	0.0
Lapse of applicable statute of limitations	(1.5)	0.0
Balance at end of year	$ 4.9	$ 3.0