Schedule II Valuation And Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Provision for Uncollectible Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$ 1,159	$ 835	$ 898
Additions	3,141	4,113	3,766
Deductions	3,247	3,789	3,829
Balance at End of Period	1,053	1,159	835
Provision for Uncollectible Accounts [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	1,159	835	897
Additions	3,141	4,113	3,766
Deductions	3,247	3,789	3,828
Balance at End of Period	1,053	1,159	835
Valuation Allowance For Deferred Tax Assets [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	38,266	39,874	40,713
Additions	4,383	0	3,501
Deductions	6,321	1,608	4,340
Balance at End of Period	36,328	38,266	39,874
Assets held for sale, current [Member] | Provision for Uncollectible Accounts [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$ 113		$ 369
Additions		2,035
Deductions		2,291
Balance at End of Period		$ 113