Supplemental Financial Information (Reclassification out of ACOI) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income	$ (31.4)	$ (33.1)	$ (93.5)	$ (96.4)
Tax expense	(0.3)	41.0	(13.5)	(29.7)
Net income / (loss)	8.6	98.4	59.0	(116.8)
Interest expense	(28.9)	(33.1)	(90.3)	(95.8)
Revenue	414.1	479.2	1,281.5	1,329.6
Purchased power	(145.4)	(153.7)	(460.2)	(466.2)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income	(12.2)	(9.2)	(30.0)	(27.0)
Tax expense	(0.8)	(13.1)	(25.0)	(23.1)
Net income / (loss)	15.5	53.2	75.9	76.5
Interest expense	(6.9)	(9.4)	(24.6)	(25.5)
Revenue	389.2	454.9	1,202.6	1,252.5
Purchased power	(142.5)	(152.4)	(452.3)	(457.3)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income / (loss)	(1.9)	3.6	(3.5)	14.6
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)	(1.2)	4.1	(0.7)	17.8
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	0.3	0.0	0.6
Tax expense	0.0	(0.1)	0.0	(0.2)
Net income / (loss)	0.0	0.2	0.0	0.4
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	0.3	0.0	0.6
Tax expense	0.0	(0.1)	0.0	(0.2)
Net income / (loss)	0.0	0.2	0.0	0.4
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	1.1	(1.5)	1.6	(7.4)
Net income / (loss)	(2.0)	3.4	(3.4)	14.2
Interest expense	(0.2)	(0.3)	(0.7)	(1.0)
Revenue	(3.8)	4.9	(7.0)	23.4
Purchased power	0.9	0.3	2.7	(0.8)
Total before income taxes	(3.1)	4.9	(5.0)	21.6
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	1.2	(1.9)	1.9	(6.7)
Net income / (loss)	(2.0)	3.2	(3.3)	15.3
Interest expense	(0.3)	(0.2)	(0.9)	(0.8)
Revenue	(3.8)	4.9	(7.0)	23.4
Purchased power	0.9	0.4	2.7	(0.6)
Total before income taxes	(3.2)	5.1	(5.2)	22.0
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]
Other income	0.1	0.0	0.3	0.0
Tax expense	0.0	0.0	(0.4)	0.0
Net income / (loss)	0.1	0.0	(0.1)	0.0
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]
Other income	1.4	1.0	4.2	3.1
Tax expense	(0.6)	(0.3)	(1.6)	(1.0)
Net income / (loss)	$ 0.8	$ 0.7	$ 2.6	$ 2.1