Supplemental Financial Information (Reclassification out of ACOI) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income	$ (27.9)	$ (29.4)
Tax expense	31.5	(0.6)
Net income / (loss)	(51.7)	31.8
Interest expense	(26.9)	(26.3)
Revenue	323.9	364.0
Purchased power	(102.0)	(121.9)
Operating Expenses	(223.1)	(143.0)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other income	(8.5)	(5.6)
Tax expense	21.9	(12.4)
Net income / (loss)	(41.8)	33.7
Interest expense	(7.6)	(5.3)
Revenue	311.1	349.2
Purchased power	(100.8)	(121.3)
Operating Expenses	(215.4)	(113.3)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net income / (loss)	1.7	(8.3)
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)	3.4	(8.2)
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	(0.1)	(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	(0.1)	(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	(0.5)	4.8
Net income / (loss)	1.0	(8.2)
Interest expense	(0.3)	(0.2)
Revenue	(1.5)	(17.2)
Purchased power	3.3	4.4
Total before income taxes	1.5	(13.0)
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)	4.6
Net income / (loss)	1.0	(8.4)
Interest expense	(0.3)	(0.3)
Revenue	(1.5)	(17.1)
Purchased power	3.3	4.4
Total before income taxes	1.5	(13.0)
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]
Tax expense	(0.5)	(0.1)
Net income / (loss)	0.8	0.0
Operating Expenses	1.3	0.1
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	(1.3)	(0.8)
Net income / (loss)	2.5	0.3
Operating Expenses	$ 3.8	$ 1.1