Debt Obligations (Current portion - Long-term Debt) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Current portion - long-term debt	$ 149.5	$ 20.1
THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Current portion - long-term debt	114.5	0.1
Pollution Control Series Maturing in January 2028 - 4.70% [Member]
Debt Instrument [Line Items]
Current portion - long-term debt	$ 35.3	0.0
Debt instrument maturity year	Jan. 01, 2028
Debt instrument interest percentage	4.70%
Pollution Control Series Maturing in January 2028 - 4.70% [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Current portion - long-term debt	$ 35.3	0.0
Pollution Control Series Maturing in January 2034 - 4.80% [Member]
Debt Instrument [Line Items]
Current portion - long-term debt	$ 79.1	0.0
Debt instrument maturity year	Jan. 01, 2034
Debt instrument interest percentage	4.80%
Pollution Control Series Maturing in January 2034 - 4.80% [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Current portion - long-term debt	$ 79.1	0.0
Bank term loan due in May 2018 - rates from: 2.41% - 2.44% and 2.42% - 2.45% [Member]
Debt Instrument [Line Items]
Current portion - long-term debt	$ 35.0	$ 20.0
Debt instrument maturity year	May 01, 2018
Debt instrument interest percentage minimum	2.41%	2.42%
Debt instrument interest percentage maximum	2.44%	2.45%
U.S. Government note maturing in February 2061 - 4.20% [Member]
Debt Instrument [Line Items]
Current portion - long-term debt	$ 0.1	$ 0.1
Debt instrument maturity year	Feb. 01, 2061
Debt instrument interest percentage	4.20%
Debt instrument interest percentage minimum
U.S. Government note maturing in February 2061 - 4.20% [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument [Line Items]
Current portion - long-term debt	$ 0.1	$ 0.1