Debt (Narrative) (Details)										3 Months Ended	6 Months Ended				9 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 08, 2017 USD ($)	Oct. 30, 2017 USD ($)	Sep. 30, 2017	Aug. 07, 2017 USD ($)	Jul. 01, 2017	Jun. 23, 2017 USD ($)	Oct. 17, 2016 USD ($)	Feb. 05, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018 USD ($)	Jul. 03, 2018 USD ($)	Sep. 30, 2017	Dec. 31, 2020	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 03, 2018	Dec. 31, 2017 USD ($)	Dec. 31, 2017 fiscal_quarter	Dec. 31, 2017 debt_covenant	Mar. 31, 2017	Dec. 31, 2016 USD ($)	Jan. 06, 2016 USD ($)	Sep. 30, 2015 fiscal_quarter	Aug. 03, 2015 USD ($)
Debt Instrument [Line Items]
Current portion - long-term debt																				$ 4,700,000
Unamortized adjustments to market value from purchase accounting																				2,500,000
Unamortized Deferred Financing Costs																				(6,800,000)				$ (8,800,000)
Current portion - long-term debt																				4,700,000				29,700,000
Repayments of debt			$ 70,000,000
Non-recurring cash expenses	$ 25,000,000
Required repayment or refinance																		$ 100,000,000
Debt Covenant, Leverage Ratio, Maximum																		0.67
Debt Covenant, Interest Coverage Ratio, Minimum																		2.50
Leverage Ratio																		1.50
Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum						0.75												0.65
Long Term Indebtedness, Less than or Equal to										$ 750,000,000.0
Total Debt to Total Capitalization Ratio	0.67			0.61						0.75					0.65
Capital Lease Obligations																				200,000				400,000
Debt Instrument, Unamortized Discount (Premium), Net																				500,000				600,000
Total Long-term Debt At Subsidiary With Purchase Accounting Adjustments																				646,800,000				747,200,000
Long-term Debt, Excluding Current Maturities																				1,700,400,000				1,828,700,000
Long-term Debt, Maturities, Repayments of Principal in Year Two																				224,500,000
THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Current portion - long-term debt																				4,600,000
Unamortized Deferred Financing Costs																				(9,800,000)				(11,700,000)
Current portion - long-term debt																				4,600,000				4,600,000
Repayments of debt			$ 70,000,000
Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum										0.75					0.65
Long Term Indebtedness, Less than or Equal to										$ 750,000,000.0								$ 750,000,000
Total Debt to Total Capitalization Ratio	0.67			0.61						0.75					0.65
Capital Lease Obligations																				0				400,000
Unamortized Deferred Financing Costs (Subsidiary)																				(9,800,000)				(10,700,000)
Debt Instrument, Unamortized Discount (Premium), Net																				2,000,000				5,500,000
Long-term Debt, Excluding Current Maturities																				642,000,000				731,500,000
Long-term Debt, Maturities, Repayments of Principal in Year Two																				4,600,000
Term Loan Maturing 2022 (DPL) [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Aug. 24, 2022
Long-term Debt, Gross																				440,600,000				445,000,000
Term Loan Maturing 2022 (DPL) [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Aug. 24, 2022
Long-term Debt, Gross																				440,600,000				445,000,000
Eurodollar rate Term Loan B [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage	3.25%									3.25%								3.25%
Eurodollar rate Term Loan B [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage	3.25%									3.25%								3.25%
Base Rate Term Loan B [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage	2.25%									2.25%								2.25%
Base Rate Term Loan B [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage	2.25%									2.25%								2.25%
Revolving Credit Agreement and Standby Letters of Credit [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Number of financial covenants	2									2								2				2
Number of prior quarters included in debt to EBITDA ratio | fiscal_quarter																					4					4
U.S. Government note maturing in 2061 - 4.20% [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Feb. 01, 2061
Long-term Debt, Gross																				17,800,000				18,000,000
Debt instrument interest percentage	4.20%									4.20%								4.20%
U.S. Government note maturing in 2061 - 4.20% [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Feb. 01, 2061
Long-term Debt, Gross																				17,800,000				$ 18,000,000
Debt instrument interest percentage	4.20%									4.20%								4.20%
First Mortgage Bonds Maturing in 2016 - 1.875%
Debt Instrument [Line Items]
Debt instrument interest percentage																								1.875%
First Mortgage Bonds Maturing in 2016 - 1.875% | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage																								1.875%
Term Loan Maturing 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross																				445,000,000
Basis spread on variable interest rate (percent)																		3.25%
Debt Instrument, Interest Rate, Effective Percentage Rate Range, Minimum LIBOR																		0.75%
Quarterly Loan Amortization of Initial Principal Balance																							0.25%
Term Loan Maturing 2022 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross																				445,000,000
Basis spread on variable interest rate (percent)																		3.25%
Debt Instrument, Interest Rate, Effective Percentage Rate Range, Minimum LIBOR																		0.75%
Quarterly Loan Amortization of Initial Principal Balance																							0.25%
Bank Term Loan maturing in May 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Jul. 31, 2020
Bank Term Loan Maturing July 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross																				70,000,000				$ 125,000,000
Extinguishment of debt, amount		$ 30,000,000
Five Year Senior Unsecured Notes At6.75 Maturing October152019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Oct. 01, 2019
Long-term Debt, Gross																				200,000,000				200,000,000
Debt instrument interest percentage	6.75%									6.75%								6.75%
DPL Revolving Credit Agreement and Term Loan Maturing July 2020 [Member]
Debt Instrument [Line Items]
Number of financial covenants | debt_covenant																						2
Debt Instrument, Debt Covenant, Debt to EBITDA Ratio, Number of Quarters | fiscal_quarter																					4
Number of prior quarters included in debt to EBITDA ratio | fiscal_quarter																					4
Senior Unsecured Bonds at 6.50% maturing in 2016 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross																									$ 130,000,000
Make Whole Premium									$ 2,400,000
Extinguishment of debt, amount								$ 57,000,000	$ 73,000,000
Debt instrument interest percentage																									6.50%
Senior Unsecured Bonds at 7.25% maturing in 2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Oct. 01, 2021
Long-term Debt, Gross																				780,000,000				780,000,000
Debt instrument interest percentage	7.25%									7.25%								7.25%
Note to DPL Capital Trust II Maturing in 2031 - 8.125% [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Sep. 01, 2031
Long-term Debt, Gross																				15,600,000				$ 15,600,000
Debt instrument interest percentage	8.125%									8.125%								8.125%
Note Payable to DPL Inc. [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage																								3.02%
Notes payable - related party																								$ 5,000,000
Variable Rate Notes Backed by Term Loan and First Mortgage Bonds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount																											$ 200,000,000.0
Pollution Control Series Maturing in 2036 - 4.80% [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Sep. 01, 2036
Long-term Debt, Gross																				0				100,000,000
Extinguishment of debt, amount					$ 21,900,000		$ 8,100,000
Debt instrument interest percentage	4.80%									4.80%								4.80%
Pollution Control Series Maturing in 2036 - 4.80% [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Sep. 01, 2036
Long-term Debt, Gross																				0				100,000,000
Extinguishment of debt, amount					$ 21,900,000		$ 8,100,000
Debt instrument interest percentage	4.80%									4.80%								4.80%
One Point One Three To One Point One Seven Bonds Maturing In August Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Aug. 01, 2020
Long-term Debt, Gross																				200,000,000				200,000,000
One Point One Three To One Point One Seven Bonds Maturing In August Two Thousand Twenty [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End																		Aug. 01, 2020
Long-term Debt, Gross																				200,000,000				200,000,000
Subsequent Event [Member]
Debt Instrument [Line Items]
Debt Covenant, Leverage Ratio, Maximum											6.75	7.00				6.50	7.25
Long Term Indebtedness, Less than or Equal to																	$ 750,000,000.0
Subsequent Event [Member] | Eurodollar rate Term Loan B [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage																			2.00%
Subsequent Event [Member] | Eurodollar rate Term Loan B [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage																			2.00%
Subsequent Event [Member] | Base Rate Term Loan B [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage																			1.00%
Subsequent Event [Member] | Base Rate Term Loan B [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage																			1.00%
Subsequent Event [Member] | Term Loan Maturing 2022 [Member]
Debt Instrument [Line Items]
Early prepayment rate														$ 1.01
Standard Repayment Rate													$ 1.00
Subsequent Event [Member] | Term Loan Maturing 2022 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Early prepayment rate														$ 1.01
Standard Repayment Rate													$ 1.00
Debt [Member]
Debt Instrument [Line Items]
Long-term Debt and Capital Lease Obligations, Including Current Maturities																				1,705,100,000				1,858,400,000
Debt [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Long-term Debt and Capital Lease Obligations, Including Current Maturities																				$ 646,600,000				$ 736,100,000