Consolidated Statements of Comprehensive Income/(Loss) (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income tax (expense)/benefit on unrealized gains (losses) related to available-for-sale securities	$ (0.2)	$ (0.1)	$ 0.1
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Tax	0.0	0.0	0.0
Income tax (expense)/benefit on unrealized gains (losses) related to derivative activity	(5.3)	(8.8)	(10.3)
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	4.4	16.7	5.4
Income tax (expense)/benefit on prior service cost related to pension and postretirement activity	0.4	0.0	0.0
Income tax (expense)/benefit on net loss related to pension and postretirement activity	1.1	2.4	(1.2)
Income tax (expense)/benefit on reclassification of earnings related to pension and postretirement activity	(0.5)	(0.6)	(0.2)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Income tax (expense)/benefit on unrealized gains (losses) related to available-for-sale securities	(0.2)	(0.1)	0.1
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Tax	0.0	0.0	0.0
Income tax (expense)/benefit on unrealized gains (losses) related to derivative activity	(7.2)	(8.7)	(10.3)
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	3.2	16.4	5.6
Income tax (expense)/benefit on prior service cost related to pension and postretirement activity	1.0	0.0	0.0
Income tax (expense)/benefit on net loss related to pension and postretirement activity	0.4	1.1	(1.0)
Income tax (expense)/benefit on reclassification of earnings related to pension and postretirement activity	$ (2.3)	$ (1.8)	$ (1.9)