Supplemental Financial Information (Reclassification out of ACOI) (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2018	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income		$ (28.6)	$ (31.5)
AOCI reclassed to Retained Earnings before tax		1.6
Tax expense		(3.6)	31.5
Net loss		16.9	(51.7)
Interest expense		(28.0)	(27.3)
Revenue		293.2	323.9
Purchased power		(97.8)	(102.0)
Operating Expenses		(112.5)	(219.5)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income		(9.5)	(9.0)
AOCI reclassed to Retained Earnings before tax	$ 1.7
Tax expense		(3.6)	(8.0)
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax		0.0	(88.7)
Discontinued Operation, Tax Effect of Discontinued Operation		0.0	29.9
Net loss		15.7	(41.8)
Interest expense		(8.2)	(7.6)
Revenue		196.0	190.1
Purchased power		(83.8)	(81.1)
Operating Expenses		(82.5)	(75.0)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net loss		1.4	1.7
Reclassification out of Accumulated Other Comprehensive Income [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net loss		(0.5)	3.4
Gains / (losses) on available-for-sale securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income		0.0	(0.1)
AOCI reclassed to Retained Earnings before tax		(1.6)	0.0
Tax expense		0.6	0.0
Net loss		(1.0)	(0.1)
Gains / (losses) on available-for-sale securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income		0.0	(0.1)
AOCI reclassed to Retained Earnings before tax		(1.7)	0.0
Tax expense		0.6	0.0
Net loss		(1.1)	(0.1)
Gains / (losses) on cash flow hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Tax expense		(0.8)	(0.5)
Net loss		2.3	1.0
Interest expense		(1.1)	(0.3)
Revenue		(2.0)	(1.5)
Purchased power		6.2	3.3
Total before income taxes		3.1	1.5
Gains / (losses) on cash flow hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Tax expense		0.5	0.1
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax		0.0	1.8
Discontinued Operation, Tax Effect of Discontinued Operation		0.0	(0.6)
Net loss		(0.3)	1.0
Interest expense		(0.8)	(0.3)
Amortization of defined benefit pension items [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Tax expense		(0.1)	(0.5)
Net loss		0.1	0.8
Operating Expenses		0.2	1.3
Amortization of defined benefit pension items [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Tax expense		(0.2)	(1.3)
Net loss		0.9	2.5
Operating Expenses		$ 1.1	$ 3.8