Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Long-term debt, earliest maturities	2016
Long-term debt, latest maturities	2061
Unrealized gains in AOCI, before tax	$ 0.5
Unrealized gains in AOCI, net of tax	0.4
Unrealized losses in AOCI, before tax	0.1
Unrealized losses in AOCI, net of tax	0.1
Unrealized gains and immaterial unrealized losses in AOCI, before tax			$ 0.8
Unrealized gains and immaterial unrealized losses in AOCI, after tax			0.5
Realized gains on investments sold to facilitate the distribution of benefits	1.0
Realized gains on investments sold to facilitate the distribution of benefits, after tax	$ 0.7
Percent of inputs to the fair value of derivative instruments from quoted market prices (percent)	97.00%
Gross additions to our existing landfill and asbestos AROs	$ 38.7	$ 1.2
Gross additions to our ash pond AROs	$ 40.6
THE DAYTON POWER AND LIGHT COMPANY [Member]
Long-term debt, earliest maturities	2016
Long-term debt, latest maturities	2061
Unrealized gains in AOCI, before tax	$ 0.7
Unrealized gains in AOCI, net of tax	0.5
Unrealized losses in AOCI, before tax	0.1
Unrealized losses in AOCI, net of tax	0.1
Unrealized gains and immaterial unrealized losses in AOCI, before tax			1.1
Unrealized gains and immaterial unrealized losses in AOCI, after tax			$ 0.7
Realized gains on investments sold to facilitate the distribution of benefits	1.0
Realized gains on investments sold to facilitate the distribution of benefits, after tax	$ 0.7
Percent of inputs to the fair value of derivative instruments from quoted market prices (percent)	97.00%
Gross additions to our existing landfill and asbestos AROs	$ 38.7	$ 1.2
Gross additions to our ash pond AROs	$ 40.6