Summary of Significant Accounting Policies (Related Party Transactions) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Sales to related party	$ 1.1	$ 0.6	$ 5.1	$ 1.8
Charges for services provided	8.9	11.7	28.5	28.4
Net prepaid/(payable) to the service company	0.1		0.1		$ (4.7)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Sales to related party	1.1	0.6	5.0	1.7
Premiums paid for insurance services provided by MVIC	(0.8)	(0.7)	(2.4)	(2.1)
Expense recoveries for services provided to DPLER	0.6	0.5	1.8	1.6
Charges for services provided	7.6	7.4	24.3	24.2
Deposits received from DPLER	2.9		2.9		20.1
Net prepaid/(payable) to the service company	0.1		0.1		$ (4.7)
DPLER [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Sales to related party	66.0	125.6	245.0	376.6
MVIC [Member] | Insurance Proceeds Received [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Insurance proceeds received	$ 0.5	$ 0.0	$ 4.3	$ 0.4