Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2018	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 0.8	$ 0.8
Other comprehensive loss before reclassifications		0.9
Amounts reclassified from accumulated other comprehensive income / (loss)		2.4
AOCI reclassed to Retained Earnings, net of tax		(1.0)
Other comprehensive income		2.3	$ 5.6
Balance, end of period		3.1
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		0.5
Amounts reclassified from accumulated other comprehensive income / (loss)		0.6
AOCI reclassed to Retained Earnings, net of tax	1.1	(1.1)	0.0
Other comprehensive income		0.0	7.2
Balance, end of period		(36.2)
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)	$ 0.0
Other comprehensive income		(1.0)
Balance, end of period		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
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		0.9
Amounts reclassified from accumulated other comprehensive income / (loss)		2.3
AOCI reclassed to Retained Earnings, net of tax		0.0
Other comprehensive income		3.2
Balance, end of period		17.9
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		0.5
Amounts reclassified from accumulated other comprehensive income / (loss)		(0.3)
AOCI reclassed to Retained Earnings, net of tax		0.0
Other comprehensive income		0.2
Balance, end of period		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)		0.1
AOCI reclassed to Retained Earnings, net of tax		0.0
Other comprehensive income		0.1
Balance, end of period		(14.8)
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)		0.9
AOCI reclassed to Retained Earnings, net of tax		0.0
Other comprehensive income		0.9
Balance, end of period		$ (37.8)