Debt (Narrative) (Details)		3 Months Ended		12 Months Ended
	Feb. 05, 2016	Mar. 31, 2017 USD ($) fiscal_quarter debt_covenant	Mar. 31, 2016 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Retirement of long-term debt		$ 7,400,000	$ 75,400,000
Total Debt to Total Capitalization Ratio		0.53
THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Retirement of long-term debt		$ 1,100,000	$ 0
Total Debt to Total Capitalization Ratio		0.53
Senior unsecured due in October 2016 - 6.5% [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.50%
Term Loan Maturing 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 443,900,000			$ 445,000,000
Term Loan Maturing 2022 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 443,900,000			$ 445,000,000
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 EBITDA to interest calculation | fiscal_quarter		4
DPL Revolving Credit Agreement and Term Loan Maturing July 2020 [Member]
Debt Instrument [Line Items]
Number of financial covenants | debt_covenant		2
Number of prior quarters included in EBITDA to interest calculation | fiscal_quarter		4
Number of prior quarters included in debt to EBITDA ratio | fiscal_quarter		4
Subsequent Event [Member]
Debt Instrument [Line Items]
Long Term Indebtedness, Less than or Equal to				$ 750,000,000.0
Subsequent Event [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Long Term Indebtedness, Less than or Equal to				$ 750,000,000.0