Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Jan. 01, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 0.8			$ 0.8
Other comprehensive loss before reclassifications				0.3
Amounts reclassified from accumulated other comprehensive income / (loss)				2.7
AOCI reclassed to Retained Earnings, net of tax	(1.0)	$ 0.0	$ 0.1	(1.0)	$ 0.2
Other comprehensive income		(0.3)	1.1	2.0	6.7
Balance, end of period		2.8		2.8
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(36.2)			(36.2)
Other comprehensive loss before reclassifications				0.7
Amounts reclassified from accumulated other comprehensive income / (loss)				1.2
AOCI reclassed to Retained Earnings, net of tax	1.1	0.0	0.1	(1.1)	0.2
Other comprehensive income		0.8	$ 1.7	0.8	$ 8.9
Balance, end of period		(35.4)		(35.4)
Gains / (losses) on available-for-sale securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	1.0			1.0
Other comprehensive loss before reclassifications				0.0
Amounts reclassified from accumulated other comprehensive income / (loss)				0.0
AOCI reclassed to Retained Earnings, net of tax				(1.0)
Other comprehensive income				(1.0)
Balance, end of period		0.0		0.0
Gains / (losses) on available-for-sale securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	1.1			1.1
Other comprehensive loss before reclassifications				0.0
Amounts reclassified from accumulated other comprehensive income / (loss)				0.0
AOCI reclassed to Retained Earnings, net of tax				(1.1)
Other comprehensive income				(1.1)
Balance, end of period		0.0		0.0
Gains / (losses) on cash flow hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	14.7			14.7
Other comprehensive loss before reclassifications				0.3
Amounts reclassified from accumulated other comprehensive income / (loss)				2.4
AOCI reclassed to Retained Earnings, net of tax				0.0
Other comprehensive income				2.7
Balance, end of period		17.4		17.4
Gains / (losses) on cash flow hedges [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	1.4			1.4
Other comprehensive loss before reclassifications				0.7
Amounts reclassified from accumulated other comprehensive income / (loss)				(0.5)
AOCI reclassed to Retained Earnings, net of tax				0.0
Other comprehensive income				0.2
Balance, end of period		1.6		1.6
Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(14.9)			(14.9)
Other comprehensive loss before reclassifications				0.0
Amounts reclassified from accumulated other comprehensive income / (loss)				0.3
AOCI reclassed to Retained Earnings, net of tax				0.0
Other comprehensive income				0.3
Balance, end of period		(14.6)		(14.6)
Change in unfunded pension obligation [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ (38.7)			(38.7)
Other comprehensive loss before reclassifications				0.0
Amounts reclassified from accumulated other comprehensive income / (loss)				1.7
AOCI reclassed to Retained Earnings, net of tax				0.0
Other comprehensive income				1.7
Balance, end of period		$ (37.0)		$ (37.0)