Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net income	$ 16.9	$ (51.7)
Available-for-sale securities activity:
Change in fair value of available-for-sale securities, net of income tax	0.0	0.2
Reclassification to earnings, net of income tax	0.0	(0.1)
AOCI reclassed to Retained Earnings, net of tax	(1.0)
Total change in fair value of available-for-sale securities	(1.0)	0.1
Derivative activity:
Change in derivative fair value, net of income tax	0.9	5.2
Reclassification of earnings, net of income tax	2.3	1.0
Total change in fair value of derivatives	3.2	6.2
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax	0.0	(0.3)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax	0.0	(1.2)
Pension and postretirement activity:
Reclassification to earnings, net of income tax	0.1	0.8
Total change in unfunded pension obligation	0.1	(0.7)
Other comprehensive income	2.3	5.6
Net comprehensive loss	19.2	(46.1)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Net income	15.7	(41.8)
Available-for-sale securities activity:
Change in fair value of available-for-sale securities, net of income tax	0.0	0.2
Reclassification to earnings, net of income tax	0.0	(0.1)
AOCI reclassed to Retained Earnings, net of tax	(1.1)	0.0
Total change in fair value of available-for-sale securities	(1.1)	0.1
Derivative activity:
Change in derivative fair value, net of income tax	0.5	5.2
Reclassification of earnings, net of income tax	(0.3)	1.0
Total change in fair value of derivatives	0.2	6.2
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax	0.0	(1.1)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax	0.0	(0.5)
Pension and postretirement activity:
Reclassification to earnings, net of income tax	0.9	2.5
Total change in unfunded pension obligation	0.9	0.9
Other comprehensive income	0.0	7.2
Net comprehensive loss	15.7	(34.6)
Gains / (losses) on available-for-sale securities [Member]
Available-for-sale securities activity:
AOCI reclassed to Retained Earnings, net of tax	(1.0)	$ 0.0
Pension and postretirement activity:
Other comprehensive income	(1.0)
Gains / (losses) on available-for-sale securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Available-for-sale securities activity:
AOCI reclassed to Retained Earnings, net of tax	(1.1)
Pension and postretirement activity:
Other comprehensive income	$ (1.1)