Debt (Narrative) (Details)								1 Months Ended	6 Months Ended	12 Months Ended
	Jul. 01, 2017	Feb. 21, 2017	Oct. 17, 2016 USD ($)	Feb. 05, 2016 USD ($)	Aug. 03, 2015 USD ($)	Jul. 01, 2015 USD ($)	Mar. 31, 2014 USD ($)	Oct. 31, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) letter_of_credit fiscal_quarter debt_covenant	Mar. 31, 2017	Jan. 06, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 fiscal_quarter	Jul. 31, 2015 USD ($)
Debt Instrument [Line Items]
Current portion - long-term debt												$ 29,700,000			$ 572,800,000
Debt Covenant, Leverage Ratio, Maximum												0.67
Debt Covenant, Interest Coverage Ratio, Minimum												2.50
Leverage Ratio												1.45
Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum												0.65
Total Debt to Total Capitalization Ratio												0.68
THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Number of Letters of Credit												2
Current portion - long-term debt												$ 4,700,000			443,100,000
Notes payable - related party							$ 15,000,000
Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum												0.65
Total Debt to Total Capitalization Ratio												0.68
Revolving Credit Agreeement with Bank Group Expiring 2018 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Unsecured revolving credit agreement									$ 300,000,000.0
Debt Instrument, Term									5 years
Letter of credit sublimit									$ 100,000,000.0
Line of credit facility, additional borrowing capacity									100,000,000.0
Letters of credit outstanding						$ 35,300,000						$ 1,400,000
Line of credit facility, remaining borrowing capacity												$ 173,600,000
Revolving Credit Agreement with Bank Group Expiring 2020 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Unsecured revolving credit agreement																	$ 175,000,000.0
Letter of credit sublimit																	50,000,000.0
Line of credit facility, additional borrowing capacity																	100,000,000.0
Revolving Credit Agreement and Standby Letters of Credit [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Number of financial covenants | debt_covenant												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												$ 18,000,000			18,100,000
Debt instrument interest percentage												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												$ 18,000,000			18,100,000
Debt instrument interest percentage												4.20%
First Mortgage Bonds Maturing in 2016 - 1.875%
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End												Sep. 01, 2016
Long-term Debt, Gross												$ 0			$ 445,000,000
Debt instrument interest percentage												1.875%			1.875%
First Mortgage Bonds Maturing in 2016 - 1.875% | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End												Sep. 01, 2016
Long-term Debt, Gross												$ 0			$ 445,000,000
Debt instrument interest percentage												1.875%			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%
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%
Bank Term Loan maturing in May 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, End												Jul. 31, 2020
Long-term Debt, Gross												$ 125,000,000			$ 125,000,000		160,000,000
Bank Term Loan Maturing July 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross																	125,000,000
DPL Revolving Credit Facility [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Unsecured revolving credit agreement									$ 100,000,000.0								205,000,000.0
Debt Instrument, Term									5 years
Number of letters of credit outstanding | letter_of_credit												2
Letter of credit sublimit									$ 100,000,000.0								200,000,000.0
Line of credit facility, additional borrowing capacity									$ 50,000,000.0								$ 95,000,000.0
Letters of credit outstanding												$ 1,700,000
Line of credit facility, remaining borrowing capacity												$ 203,300,000
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]
Debt Instrument, Maturity Date Range, End												Oct. 01, 2016
Long-term Debt, Gross								$ 130,000,000				$ 0		$ 130,000,000	130,000,000
Make Whole Premium				$ 2,400,000
Extinguishment of debt, amount			$ 57,000,000	$ 73,000,000				$ 300,000,000
Debt instrument interest percentage								6.50%				6.50%		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%
Senior Unsecured notes maturing October 2019 at 6.75% [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount								$ 200,000,000
Debt instrument interest percentage								6.75%
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
Repayments of debt								$ 5,000,000
Debt instrument interest percentage												8.125%
Pollution control series maturing in November 2040 - variable rates: 0.04% - 0.26% and 0.06% - 0.32% [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Extinguishment of debt, amount					$ 100,000,000
Pollution Control Series Maturing in 2028 - 4.70% [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Extinguishment of debt, amount						$ 35,300,000
Debt instrument interest percentage						4.70%
Pollution Control Series Maturing in 2034 - 4.80% [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Extinguishment of debt, amount					137,800,000	$ 41,300,000
Repayments of debt					$ 37,800,000
Debt instrument interest percentage					4.80%	4.80%
Note Payable to DPL Inc. [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt instrument interest percentage												3.02%			2.67%
Notes payable - related party												$ 5,000,000			$ 35,000,000
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
London Interbank Offered Rate (LIBOR) [Member] | Note Payable to DPL Inc. [Member]
Debt Instrument [Line Items]
Basis spread on variable interest rate (percent)							2.00%
Subsequent Event [Member]
Debt Instrument [Line Items]
Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum	0.75
Long Term Indebtedness, Less than or Equal to										$ 750,000,000.0	$ 750,000,000.0
Total Debt to Total Capitalization Ratio		0.53
Subsequent Event [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum	0.75
Long Term Indebtedness, Less than or Equal to										$ 750,000,000	$ 750,000,000
Total Debt to Total Capitalization Ratio		0.53
Subsequent Event [Member] | Term Loan Maturing 2022 [Member]
Debt Instrument [Line Items]
Quarterly Loan Amortization of Initial Principal Balance													0.25%
Subsequent Event [Member] | Term Loan Maturing 2022 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Quarterly Loan Amortization of Initial Principal Balance													0.25%