Supplemental Financial Information (Reclassification out of ACOI) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income	$ (28.2)	$ (26.0)	$ (56.1)	$ (55.4)
Tax expense	(8.3)	88.3	23.2	87.7
Net income / (loss)	0.5	(136.8)	(51.2)	(105.0)
Interest expense	(27.4)	(26.0)	(54.3)	(52.3)
Revenue	298.1	328.3	622.0	692.3
Purchased power	(77.4)	(96.9)	(179.4)	(218.8)
Operating Expenses	(129.7)	(370.3)	(352.8)	(513.3)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income	(8.2)	(5.2)	(16.7)	(10.8)
Tax expense	(10.2)	303.7	11.7	291.3
Net income / (loss)	16.1	(531.6)	(25.7)	(497.9)
Interest expense	(7.8)	(5.4)	(15.4)	(10.7)
Revenue	284.1	313.7	595.2	662.9
Purchased power	(76.4)	(96.7)	(177.2)	(218.0)
Operating Expenses	(123.3)	(991.5)	(338.7)	(1,104.8)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income / (loss)	(0.1)	(10.6)	1.8	(19.0)
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 income / (loss)	0.5	(10.0)	4.0	(18.2)
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	(0.1)	0.0
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	(0.1)	0.0
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	0.0	5.9	(0.5)	10.7
Net income / (loss)	(0.2)	(10.8)	1.0	(19.0)
Interest expense	(0.3)	(0.3)	(0.5)	(0.5)
Revenue	0.1	(20.1)	(1.3)	(37.3)
Purchased power	0.0	3.7	3.3	8.1
Total before income taxes	(0.2)	(16.7)	1.5	(29.7)
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	0.0	5.9	(0.5)	10.5
Net income / (loss)	(0.2)	(10.8)	0.9	(19.2)
Interest expense	(0.3)	(0.3)	(0.6)	(0.6)
Revenue	0.1	(20.1)	(1.3)	(37.2)
Purchased power	0.0	3.7	3.3	8.1
Total before income taxes	(0.2)	(16.7)	1.4	(29.7)
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	0.0	0.0	(0.5)	(0.2)
Net income / (loss)	0.1	0.1	0.9	0.0
Operating Expenses	0.1	0.1	1.4	0.2
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	(0.3)	(0.4)	(1.6)	(1.2)
Net income / (loss)	0.7	0.7	3.2	1.0
Operating Expenses	$ 1.0	$ 1.1	$ 4.8	$ 2.2